I just generally dislike Rock and Dark-type Pokemon. Ground-types are like a sub division of Rock, with similar weaknesses and resistance, but much more strength. It defeats Flying-types better than Ground-types I admit, but so can Ice-types, plus Flying-types have a lot of neutral weaknesses anyway.
Dark-tpes seem overrated. A minority, or maybe more, of the players find Dark-types to be "cool", which is fine, but it's whole induction seems unnecessary, much like Steel-types (although I love Steel-types personally). They're resistances and weaknesses are also obscure and partially unbalanced which puts me off even more.
And from looking at the poll so far, why is Poison and Bug-types the most hated? Poison-types are great, although a little obscure, but with D/P's collection of Poison-type moves, I think they're fantastic. Bug-types I admit could be hated, since I hate bugs as well, but in-game wise, they're sub-par at worst, not bogey-able.