1) KareKano was one of the most boring anime 
series I ever watched. It was something like a 
Japanese version of "Dawson's Creek", and 
Dawson's Creek isn't good either. KareKano was 
the first anime series I stopped to watch after 
episode 10 because it was so damn boring (but a 
friend of me even couldn't stand more than 2 
episodes). 

2) Hacksign: As said by some posters above, also 
I think it was a big disappointment. 

3) DigiCharat: I don't know why people like it, I 
just couldn't stand watching more than 2 
episodes. The only watchable thing in the series 
was Rabbi-en-Rose (because she's cute ^^). 

4) Angel Sanctuary: Only seen 3 episodes, but 
it's just weird and far too depressive...