Well something like Love Hina where the character grows and the show is funny is a good harem anime for me. Fanservice doesn't hurt.
Most harem anime SUCK. I'm not talking about Shounen actionriffic with plenty of girls. I'm talking about the shows that the HAREM is most of the anime. Everything from Futakoi, Clannad, to Shuffle!.
It's made for virgin otaku's to live out their fantasies through the 2D. So if they don't have a good story with interesting characters to go behind it like Love Hina, enough fanservice to make it borderline hentai like Ninomiya-kun for the fappage, or make it funny enough like Sayonara Zetsubou Sensei where the harem is secondary the harem anime WILL SUCK.
Clannad wasn't a good anime. Visual Novels belong on /jp/ and they have a whole different set of standards than an anime for me at least.