Reddit has one of the worst interfaces ever but still has insane amounts of traffic.
I've heard from some people that Wikipedia's UI is pretty bad.
I think Quora's UI is bad as well.
So I guess this brings up the question of how important design is for products. Reddit & Wikipedia are hugely successful because they are unique in the type of content that is available on them - people are forced to use them because they have no other alternative.
Without a doubt, the QWERTY keyboard. It looks like someone came home drunk one night, knocked over a scrabble board and then tried to rearrange the letters in the dark.
A couple of sobering facts: