I haven't quit an anime series, yet. When I start a new series I always want to make sure that I'll like it, so I pick the ones to which I can relate, that have a good story/theme or both. Plus I always stick with short and moderate length series, simply because long ones (Naruto. Bleach etc) tend to be piled up with fillers and begin to, either change too much for the worse, or loose quality in animation, story, characters etc.
Btw, I watched Haruhi through Endless Eight, so I generally oversee some negative aspects, as long as they don't conflict with my principles and concepts.
Regarding Clannad:
I have never seen anything, nor do I think I'll ever again see somehing as superb, pure and downright beautiful as Clannad. I filled buckets with tears of sadness, and I cried just as many buckets of happiness when I saw the conclusion of the series and the true end of the VN.