I'll jump into this for a second and contribute.
Personally I don't have a problem with OHK type spells/creature abilities. In fact I fully expect and hope we DO have to deal with at least some. After the Spectator fight I will be seriously let down if the late/end game does not feature a full on Beholder battle at some point, which has more than 1 type of potential OHK if I recall correctly.
While I would not call for it as part of the main quest line, I'm fine with a side quest or extra fight that is next to impossible except under specific circumstances or that only has a limited number of strategies that can successfully overcome it. I fully believe that there should be some things in the game(once more probably not as part of the main quest for the sake of the average player) that are just there for bragging rights and a really tough fight.
D&D has a ton of kewl monsters and enemy types and I'd like to see as many neato ones as possible(preferably as close to the monster manual as possible, hint hint Larian please stop homebrewing the creatures)...even if they are a bit higher up the food chain than ourselves.
