Berserk. The first story arc is nowhere near as well drawn as the rest of the series, the main character comes off as the epitome of juvenile "ultimate grimdark super badass" power fantasies, the darkness and the gore go so far over the top it switches between funny and insulting, and the pacing isn't really very good. Good thing the rest of the series is sublime then.
I guess you could count the Spartacus series for the same reasons as well, but when I watched the first episode I knew what I was in for, so it didn't really leave a bad taste in my mouth. It was so bonkers and over the top it was actually funny, and I pretty much laughed through the whole first episode.
Fate/Zero. I even made a thread about whether or not it was going to get any better after 4 episodes. I never made it past episode 6. I had no idea what was supposed to be going on, what the setting was like, what the rules of the world were, or most of the time even what the characters were talking about. It felt like I was watching some weird anime Diablo fan fiction.