I'm speaking for a DM of mine, but I
know he hates the Fighter's Combat Superiority. It has shut down more Big Nasty monsters from fleeing than anything else.
Also when the wizard drops Wizard's Fury, the combat is typically over before it starts.
As for me, probably defender lockdowns and anything that prevents a monster from moving/attacking. Namely because I know the monsters are going to die ASAP, so I want to hurt the characters as much as possible to make it challenging. Such as the Pursuing Avenger, since many monsters have "shift away" powers.
I hate it when a monster just doesn't get to pose a threat. That includes missing on all its cool stuff.
Honestly? I hate it when a PC picks a really poor power and then I have to watch them use it, to little or no effect, or not use it at all (which is even worse) until they level up and retrain it.
Then why be so strict with the retrain rules? I have a philosophy that if the player picks something and then realizes how poor it is, then it's only
fair to say "Hey, just pick something else after this session." Retraining IMO is useful when "This power/feat is no longer useful to me" or "I'd rather get this new shiny thing than this thing I used to have".
But then, I let a player just switch his class mid-adventure (the character went from an Infernal warlock to a fire-based Sorcerer, while retaining the same background and the RP aspect of the Pact) because he wasn't as satisfied with the warlock.