I have been trying to think of a simple reply to this but am struggling because it is such a broad question!
One thing that usually annoys me is when there's a few.. say 3 or 4 tracks in a row in the same key.
Album length I suppose for me is not a major issue, if the songs were all very similar I wouldn't be able to sit through a long album, but if they were good solid contrasting songs AND arrangements (as well as of course, in a style I liked) I could listen to it for a long time. I think anything from 8-12 tracks works well if they are on average your standard 3-4 minute songs.
One of my favourite albums is Patrick Watson's 'Adventures in your own backyard'. I think Patrick Watson (the band and the guy) are fantastic and they use lot's of different sounds. Also, the songs aren't overly similar to each other, though all well written/produced. it has 12 tracks. I don't get bored in the slightest listening to it the whole way through.
I have many favourite albums which are not similar at all, though. I like to sit down and listen to a full album when I can find the time.
This is a tough one. I don't know if my answer is even an answer, but it's my thoughts!