Mottled dice where the pips are indistinguishable with a glance, tiny dice, and dice where the 6 and/or 1 is a symbol are my pet peeves. Most of those aren't a problem in isolation, it is when we have multiple levels of dice silliness that it becomes an impediment to the game. Mottled tiny dice where the 6 is a symbol on half and the 1 is a symbol on the other half, for example.
Your dice aren't cool enough to justify being a pain in the ass for your opponent.