I generally don't like vampire shows all that much -- whether they be live action or animated. I think it has something to do with the fact that most stories in that genre try to make the violence moody if not romantic, and that just doesn't work for me.
But Hellsing seemed like it would be different. And for the first half of the show, it was different. The violence was action-oriented and while the characters got excited by the violence, it was more like adrenaline-pumping euphoria as opposed some kind of moody romance. Plus, the characters were either just plain nuts (Alexandar), or they had the perpetual confident/cocky smile that made them seem so cool (Alucard).
But then a couple things ruined the show. First off, it got way too moody. Secondly, it dumped one of the best characters -- Alexandar -- for some stupid new villian named Incognito. And thirdly, it never even wrapped up the major driving plot. It just left it hanging.
I know there are plenty of fans of this show out there, and I would have been one of them if the second half kept pace with the first half. But in the end it disappointed.