Ok, I'm not normally one to reply to these types of threads but honestly, there is only one key to being bad/good in bed....
Communication. When I've had a bad partner it is because they were too selfish to really listen to my needs or my wants in order to please me. Most girls need to be "warmed up," IE: touching, kissing, making out. You can't just go up and stick it in!
Same with guys, there is normally something that will get them aroused and get them interested.
So, when a guy is bad in bed, he's not listening...
(And no, I do not believe that the size of a penis matters. :-O I know! But it's again how they use it, and figuring out different ways, positions that will get both partners pleased...)