Well, given that it's unlikely to flip the channel and run into some piss poor anime one never meant to seek out, I'm not sure there's much purpose to warning people which anime series to actively avoid.
Still, I suppose I'll warn against [http://www.escapistmagazine.com/forums/read/326.302761-Anime-Review-Higurashi-no-Naku-Koro-ni-Kira-OVA-1] the Higurashi no Naku Koro Ni Kira OVA's. Because you can never warn too much about those. Two first seasons and the most recent OVA is all anyone should ever concern themselves with.
Still, I suppose I'll warn against [http://www.escapistmagazine.com/forums/read/326.302761-Anime-Review-Higurashi-no-Naku-Koro-ni-Kira-OVA-1] the Higurashi no Naku Koro Ni Kira OVA's. Because you can never warn too much about those. Two first seasons and the most recent OVA is all anyone should ever concern themselves with.