I've seen a couple that were so bad I didn't
bother remembering the names of them *l*.
  
Here are some that either disappointed me or lost
my interest:

Tenchi Muyo - I watched quite a bit, but I never
liked it enough to keep watching.  It never really
caught my attention enough for me to, which is
very odd because I usually have a pretty high
tolerance to new anime.

Akira - I didn't mind the artwork, but all the
characters look so much alike it was a little
confusing in the beginning.  I had seen that so
many people liked it, but after I saw it I was
very disappointed.  I wouldn't say that I disliked
it though.

Weiss Kruez - Another one I had heard good things
about, I only have watched the first dvd so I
haven't made up my mind and I will give it another
chance, but I was pretty disappointed.  I really
love the character designs though.

Digimon & Pokemon - I watched some episodes but
both series just aren't my cup of tea *l*

Ah My Goddess - I can't say much about this one,
it bored me so much I fell asleep watching it...

When I first watched Dragonball Z I loved it.  I
still do, but as another said, after the Frieza
Saga I was disappointed, and I got more and more
disappointed the more I watched.  The story got
pretty bad around the Majin Boo Saga I really
hated the change in artwork.  I read once that
Akira Toryama lost a lot of interest in the series
after the Frieza Saga..

I have to disagree with some series listed by
others though, since I guess this post is opinion
based I'll throw my two cents in as well.  I
really love Trigun, Inu Yasha, Cowboy Bebop and
the Kenshin Series - TV and OAVs.