Man, looking over this thread makes me seem a lot less picky than I thought I was, lol.
I think that most actors can be good in certain roles, but it becomes annoying to me when an actor is put into roles they aren't suited for.
Tom Cruise is a good example. I actually don't mind him when he's playnig a jerk and/or villain character (I thought he was great in Rain Man in particular), but he's proven that he can't play any sort of good guy or hero role decently if his life depended on it.
Same pretty much goes for Brad Pitt, Angelina Jolie, and Leonardo DiCaprio. They're not necessarily bad in every role they've ever played, but they got too popular and get put into roles they're not suited for based on their name recognition alone. I am sort of interested to see Angelina play the evil queen in the upcoming Malificent though, I'm not really sure what to expect from her in that.
There are very few actors to me that seem genuinely bad at acting altogether... though I will say I think Kristen Stewart and Megan Fox may be two exceptions that do fit that bill.