I have fall in love with at least one of the major characters. That means good acting plus great script. I also like when the lines are witty you want to pause sometimes and note them. I don't like when there are too many unnecessary scenes that seem to be added to drag or make the series longer than it should be.
I like shows that are somewhat philosophical like House or Without a Trace. Also, of course, good acting and an interesting plot. Oh, and there should be at least one overwhelmingly good looking woman.
Drama shows have to be able to mix in humor and vise versa. It has to be well written and acted. I think that comedies and cartoons can get away with a weaker plotline but I see continuity as being very important.