I used to say "maybe it's not a bad show, give it a few more episodes" and then I was four episodes away from the end and I hated every character in it. I also used to watch anything with pretty artwork. The only shows I regret watching are the ones which names I can't recall for the life of me, since they were as generic as they come. I mean the "seen one, seen them all" harems, the "came back to the country and can't remember the promise I made to my osananajimi" shows, and also, most shounen-ai out there (I'm looking at you, Junjou Romantica and Sekaiichi Hatsukoi). Other than that, there haven't been any shows I have truly wished I never saw --- maybe the second half of Guilty Crown, but that's it.