For me, a unique premise and story is important to my enjoyment of an anime. Variety is the spice of life, and the best way to catch my interest is to suggest a new idea or concept that I haven't really thought about before.
Music and atmosphere are also extremely important in creating an anime that draws me in. For me the colours used and the music playing have very significant roles in whether or not I'm engrossed in the anime. I'm a sucker for rain and dark colours with classical instruments, but there are lots of brighter scenes I've enjoyed.
I think that's really the biggest things I look for. Obviously good voice acting is nice, but I'm rarely let down by Japanese animations in this regard. They do a good job. I enjoy seeing different art styles, so I'm not really picky about that. Genre is largely unimportant to me as long as it is moderately intelligent. I won't watch hentai/harum or whatever. I tend not to like giant fighting robot anime, but that's really because most of them are very cliche and boring, not because I don't enjoy a well done one.
Things that ruin an anime for me are excessive fanfare or general perversion for no reason. I like to be able to share my thoughts on the intelligent aspects of an anime, and these sometimes make me embarassed to even admit I've watched it, not to mention delegitimize my hobby. Of course there's artful nudity and I don't really care if there's a bathhouse scene, I just don't need every dramatic moment to be ruined by a strong gust of wind and a stupidly short skirt. It doesn't help the story and it's obviously done for cheap ratings... Which is something I don't respect.
Another thing that ruins an anime for me is the DBZ/videogame level up mechanic. I don't like arbitrary forms of power that just mean more wind and more yelling and longer scenes of wind and yelling. That isn't entertaining, and it's also cliche. I feel like it's a copout for the author to actually think of something that would give a person power. Good: Death Note. Bad: Even thicker plated armour. Good: Newfound vampiric powers. Bad: Bigger ship.
As far as length goes. An anime should only be as long as its plot. I don't really care how long that is (okay, over 100 episodes is probably too much for me, but I can't think of any good plot that's gone that long). Filler episodes will ruin a good anime, but an anime who's plot never stops moving will apply greatness to a good anime. I think I'd apply this thought most to Monster. That series had so many filler episodes that I even begun to dislike it, but by the time I finished it I realised it did have a good plot... It was just stretched way too thin. Some people might say Death Note should have ended 10 episodes earlier, just because they dislike the WAY the plot went, but it did keep every episode action packed and there's a reason everyone seems to recommend it. Monster should have been about 30 episodes shorter just because it didn't use them for plot. I don't need 10 episodes of Tenma walking old ladies across the street to know he's a nice guy.
That's my pretentious opinion
