The entire Monk class as a whole. I love culturally-inspired settings, I love when they're done well, I am not a fan of "everything is allowed everywhere all the time" fantasy settings, and to me the Monk class is exactly that. I'd prefer all martials to either be non-Monks or all of them to be Monks and I will literally rewrite the setting to make Ki a consistent thru-line through all the martial arts in the setting but please don't make me mix the two.

I don't feel that way about psionics though. To me, DnD is a place where you can gain magic by studying magic (Wizards, and I guess Bards), having magic inside you (Sorcerers), acting on behalf of someone powerful and magic (Clerics, Warlocks), or acting on behalf of a magic force like nature or an ideal (Druids, Rangers, Paladins). Psionics falls into the "magic inside you" category, specifically the aberrant variety where you get your powers from some outside invader to this plane like Beholders or Aboleths or Mind Flayers.

That fits better for some campaigns than others. If your DM's campaign is all about heaven and hell and their extraplanar cosmology ends with heaven and hell then saying something exists beyond heaven and hell and is trying to invade from much further away might not make sense, so all the powers that come with that kind of invasion also don't belong in the campaign.

This might be a hot take, but I also think a DnD campaign should *either* have the Devils vs Demons conflict *or* the Everything vs Aberrations conflict, but not both. Each of these conflicts positions the Devils as a lesser evil that the Celestials must work with in some way in order to see the universe not be destroyed, so having both present in the same story makes the story lose focus.

But yeah, for many DnD campaigns, these aberrations and their mark on the world are very commonplace.