The biggest single problem I saw on online dating profiles for girls: They go on big tirades of things they demand in a man (the lists are often lengthy and silly); at no point do they mention what THEY bring to the table in the relationship.
Problem girls usually fit in one of these categories:
1) Crazy
2) Typical "young, dumb, and looking for douche-bags" syndrome. They'll swear up and down that they want nice guys, but their track record shows otherwise.
3) Perfectionist
4) Bridezilla ego
The hilarious thing? The men on those sites are apparently worse
The Romeos there want to tell women about their POS car, drug habits, or show them pictures of their skinny man abs in a mirror.
I don't know how many times I got a conversation with a woman simply for: speaking English, not referring to her as a pet name, not speaking to her in a derogatory way, or making choice comments about her appearance. Ridiculous.