I think spells are a big problem - I remember letting a 1e/2e player persuade me his Wizard could have "Elminster's Evasion" *ugh* 
My rule is that everything outside the PHB will be evaluated on a case by case basis, and disallowed if it conflicts with what I want in my campaign (which is gritty swords & sorcery feel). Stuff in the PHB, mostly spells, is subject to change if I find it causes a problem. This seems to work well. A player can buy player's option books for suitable ideas (fluff) but the crunch is always going to be looked at & changed.

My rule is that everything outside the PHB will be evaluated on a case by case basis, and disallowed if it conflicts with what I want in my campaign (which is gritty swords & sorcery feel). Stuff in the PHB, mostly spells, is subject to change if I find it causes a problem. This seems to work well. A player can buy player's option books for suitable ideas (fluff) but the crunch is always going to be looked at & changed.