I don't think you should tell a person "what's wrong with them" but why your relationship with them wasn't working out. For instance, perhaps you two are going in different directions in life. Things that you used to enjoy together are no longer fun. Maybe this is something that you two can work out, and maybe it's not. Maybe you just want out for other reasons. I thinks it's easiest to be honest with someone. You don't have to tell them that they suck in bed or other mean comments like that, but atleast come to terms with them about why you think the relationship should end.
There's my $0.02....
