Very good article on main ux mistakes.
1: Designing your site navigation to match internal corporate structure (#user centred design)
2: Not playing nice with other online services (#use existing services & functions)
3: Building a lame, lame, lame social network (#use existing networks)
4: Not knowing when to ignore and when to listen (# reasearch constraints)
5: Asking people what they think (#opinions vs. behavior)
6: Missing the point about UX (#good functional experience)
7: Letting product marketing people run the show (#Marty Neumeier)
8: Holding the user’s hand (#help not force)
9: Making content that looks like advertising (#integration)
10.Forcing visitors to register to access basic information (#bounce)
If you didn't know already or want to read them in detail have a look here.