I'm okay with character choice nerfs during pitch / session 0 for thematic or game-play reasons. For example, "In Dark Sun, any arcane caster is going to be looked on with fear and revulsion if found out, and casting normally will 'defile' and you will need to spend character resources not to kill plants and life around you." I would be perfectly fine with. Or "I don't have a high pace of encounters per day, I'm running gritty rest variant so you'll basically only have a long rest between adventures." A-Ok to me, gives me something outside normal parameters which is fun.
When you get DMs saying " Way of the Four Elements Monk is OP, I'm adding two to all of the Ki costs" - as long as I know ahead of time I can just avoid the DM's pet peeves.
I play with a great DM but he likes to keep time pressure going so one the action starts there's very rarely a chance to have a short rest. It's "end of long rest -> all action -> short rest -> long rest". So classes like the Warlock or Monk or to a lesser degree Bards and some Fighters don't get recharges mid-action. It's just how he likes to run it, but it nerfs those classes compared to others.
(And just to finish the story, I talked to him about it like a human being, gave examples from the last few sessions, he said he'd think about it, followed up with him a few months later and we went through the sessions since where the same thing was happening, and he admitted he saw the pattern, but he like the pacing so instead changed short rest to 5 minutes but maximum twice a day per character which was an acceptable compromise.)
What I can't stand is knee-jerk reactions that lead to mid-session nerfs. I don't particularly like well thought out nerfs to existing characters that are discussed calmly between sessions, but since I've offered to "de-tune" an overly optimized character in 3.x to match the group optimization more than once, I understand that it can happen mid-campaign. But just breaking it out in the middle of a session that your character doesn't work like "that" anymore would really annoy me, even if it wasn't my character.
This case of "moon druid is OP, here's an alternate you can use" presented befroe start I wouldn't really have a problem with it. If we had done a Session 0, I mentioned Moon Druid then and the DM was quiet, and other people were making characters assuming I was making that moon druid I would feel annoyed they hadn't told me sooner, but still okay with it. (And as a personal side note, Moon Druid rocks at 2nd, but then drifts back into the pack and is reasonable.)
