Id like to know how you deal with Powers, that have positive or negative sideeffects on you / your party. Do you tell your GM right away or wait untill it takes effect.
For example, you use a power that, as a sideeffect, cancels every forced movement on yourself. Do you tell your GM, that you cant be pushed until your next turn or do you wait until an enemy trys to push you and say "Aha! that wont affect me."?
The reason why i am asking this, is that some GM´s tend to avoid using powers, if they know, it wont effect the player (somewhat like: "Ok, i know i cant bullrush him this round, so i just hit him with a heavy blow.").
What is your GM like, what whould you prefere and how do you describe such effects if you trigger them?
I've always told him, as it makes things easier in the long run, especially as it's prolly reasonable for certain monsters to be aware of these effects.
Technically the monsters should know or you could give some clue in a description.
However, when I'm DM-ing, I'm OK for the players to keep a few secrets like this up their sleeves (though I do have access to the character sheets, so I could find out if a wanted anyway). I think it's fair game for them to have a small moment of glory and surprise both me and the monsters.
After all, I'm forever doing this as DM when monsters use their conditional powers for the first time. For example, last session I just an encounter with enemies using "Speed of the Dead" (refluffed in my case, it grants +2 to hit and +1d6 damage to Opportunity Attacks) and the Artful Dodger Rogue tried to showboat and moved away from 3 foes . . . ouch for her, she didn't finish her turn . . . 
. . . no-one risks OAs for the rest of the encounter!
So my advice if you are not sure is to check with the DM. Or simply do it once to see whether the DM takes it in good spirit.
I would say it depends on the DM. My group, including the DM are very good with keeping OOC and IC knowledge apart, meaning it's not an issue to remind him, but at the same time some stuff don't need to be said since it won't make a difference ingame anyway.
…or do you wait until an enemy trys to push you and say "Aha! that wont affect me."
That would be an incredibly infanitle and antagonistic thing to do.
I am on the fence for this one. If you apply any condition to an enemy you have to make sure their controller knows. aka If you mark with your Paladin you would have to make it clear they attack anyone else and they get -2 and take x radiant damage or if the DM puts an effect on your character he has to let you know what the condition is and does. When you apply conditions to yourself though.
I know that subconsciously people would shy away from taking actions that your effect would prevent or mitigate. Just natural. When I DM I sometimes make conscious efforts to make sure at some point a monster does something that a player can go haha I take less damage because of x or he tries to move me but I dont. Its like, if you are DMing and some player has a dwarf still making sure if an enemy has a forced movement that they dont ignore the dwarf.
I always do so, although I've been accused of sounding like a Yu-Gi-Oh! dub character when I do it.
Most of us know the game well enough that we can't get away with surprising the players/DM even if technically the characters/baddies don't know what's going on. In situations where one of us has dug something up that we think will be a surprise, we describe the effect (i.e. "Suddenly I'm engulfed in blue flames that emit no heat!") and no one asks what's going on because, presumably, they will find out. We enjoy it this way because we aren't often that surprised by powers and effects anymore. Also, for our group, it's nice when the DM doesn't know everything and has to base monster tactics based on what they're seeing. (i.e. "Hmm, blue flames that emit no heat...well...even so, they're afraid of catching on fire so they will all back off and their archers will focus on you!")
As a player I give the DM any relevant information on the power so he knows what will happen. As a DM I expect players to tell me everything their powers will do, however, plenty of monsters will ignore penalties based upon their intelligence or anger.
For example, running a large zombie brute with a 1 intelligence. It couldn't care less about the consequences of anything it does. I played it as regularly ignoring marks, harmful effects, etc. and would typically have go all out after the nearest PC or the one which most recently damaged it. In contrast, a Drow Warlock I ran would regularly try to go after the weakest PC's, separate herself from Defenders direct her underlings to screen her from Strikers.
I guess I would say PC's should get to use their HA! moments on enemies, not on the DM. The DM should be aware of them because some enemies will be more aware of them than others and it shoulud be up to him whether the enemy is smart enough to let the PC get away with it or not. My 2 cents.
You're DM will find out any ways. You may surprise him the first time, but you're powers don't change as fast as his.
Though i agree that the monsters shouldn't always know. It would be the same as if the player looked up monsters in the MM.
The DM should definitely be aware of all of the characters' abilities and actions. How the DM uses this information should depend on the context. If a power targets a creature, for instance, that creature does gain immediate knowledge of the effect imposed upon it, and it would then be perfectly resonable for the DM to adjust the monsters' tactics accordingly. If a power or ability does not target a creature and merely has a personal effect, then the DM should still be made aware of the mechanical effect, but it would generally not be reasonable for the DM to then turn around and use that knowledge to meta-game with the monsters' tactics.
I always tell my DM about side effects, but I'm currently playing a Pacifist Healer. Everything I have is side effects. He sometimes looks at me and flips me the finger, but we're all still having a good time.
It's also gotten to the point where he asks me for reminders (since there can be so many) or points out to the other party members why something did/didn't work. He's really good at having the monsters (or bad guys) react in appropriate ways for their character, so telling all the side effects outright doesn't cause a problem.
Here is the trouble with keeping things from the DM and suprising them with it. Some effects would be visible or known to the creature, and in effect the player is trapping the monster into taking the wrong action when wouldn't normally.
Inteligant creatures especially might recognize and spot different spells and abilities and would expect those particular results. Keeping them from the DM is in effect cheating the monsters of their passive senses and knowledge in that respect, and the player is thus forcing them to fail where they could otherwise not.
This also consumes additional time in battle, as the DM has to rework the NPC's actions after the effect, make additional rolls etc to accomidate the sudden change in what the monster does, when it suddenly can't do what he has just worked out that it had done.
It's also being rude to the DM and is underminding him. Basically you are accusing him of cheating and playing unfairly (when in fact it is you who is hiding things from the DM) and are thus taking matters into your own hand. If you have issues with the DM meta-gaming then you should discuss these with the DM outside of gameplay, but spinging your solution to teach him a lesson or to force him to play "fair" is not imo acceptable.
I know I would be very irate at a player who was doing this, and would be very put out by it.
From the PHB, page 57: "Whenever you affect a creature with a power, that creature knows exactly what you've done to it and what conditions you've imposed. For example, when a paladin uses divine challenge against an enemy, the enemy knows that it has been marked and that it will therefore take a penalty to attack rolls and some damage if it attacks anyone aside from the paladin."
The DMG also talks about this, on pages 26-27. The DM should give the players enough info to know what do, and what not to do. In particular, it talks about "Gotcha abilities", and staying away from them. Surprises can be fun. Not giving the players info that would affect their actions isn't fun. What you want to stay away from is someone making the comment "well, if I'd known that I wouldn't have done what I did..." It's the old "save or die" syndrome, you have to be careful with things like that.
Our group does an ok job with RPing stuff so it is more cinematic and less mechanical. When the warlock uses Dire Radiance, he says "The gnoll takes x damage. A glowing nimbus of lights surrounds it, and it gets the sense that if it moves closer to me the light will burn him for more damage." When the fighter marks someone, he says "I call out my combat challenge on the orc, insulting him and several generations of his mothers before him. If he tries to attack anyone else, he'll be so distracted that he'll have a hard time hitting them, and he should probably focus his anger on me." Etc.
Everyone knows what mechanics are involved, so we try to describe it more interesting ways. If it's unclear, or if it's a new power, we'll read it out loud so everyone knows what's involved. Particularly if there are other effets like granting Combat Advantage or things like that, other players will want to know!
…or do you wait until an enemy trys to push you and say "Aha! that wont affect me."That would be an incredibly infanitle and antagonistic thing to do.
I have to agree. I often intentionally use ineffective tactics against the PCs so they can feel awesome, or I forget about certain abilities and do so unintentionally. I'm happy doing either, but I'd get annoyed with a player who went out of his way to get gloat time.
As far as i see it, many of you deal with powereffects the way we do in our party (describe it in RP-manner).
Never the less id like to keep a bit more focused on the question. Maybe my example was a bit to provocant so some of you overlooked the type of effects in question:
Negative effects on monsters, have to be told to the GM, as some of you allready confimed by the book (PHB, page 57). My question centered to powers, that have positive or negativ effects on the players, like: reduce damage, grant bonus to savingthrows, negate push/pull/slide and also grant CA, get weakend, loose hp....
I am especially interested in how visual those effects shall be described for the NPCs.
Most fights against (unknown)monsters feel like a round of poker, against NPCs with a perfect pokerface, while you keep you cards head up on the table in front of you.
Maybe to improve my question a bit for the GMs out there: What do you tell the players about monsterpowers? Should the players know about damage reduction, after hitting the monster? Do they know if the power a monster just used is an encounter, an at will or a reloading skill? Do they know if it recharged this turn?...
Somewhat a question about "give and take" between NPCs and PCs.
I usually tell our DM when I have certain advantages over the enemy.
Though I do tend to keep a secret or two up my sleeves. Such as when I have higher chances of hitting with an opportunity attack, when I gain bounses after killing stuff or something else.
Also, I usually keep it secret what attack I'm using, when I use Paladin's Judgment (also referred to as Strike of Truly Unquestionable Accuracy), since I never hit if I say that I'm using that attack.
Typically I don't tell him. Mostly because I assume when i say "I use the power of MAGIC MISSLE!" (replace MM with anything) I assume he knows. IF he doesn't he will ask, and i will tell.
Whenever i use my Noose power for my assassin though, I always tend to say "He is pulled two and slowed." Often with a bit of a cheshire Cat grin because i know that just screwed up that damn deathjump spider.