It tends to make me curious if the cover artwork seems incongruent with the genre. That usually makes me take a listen. I use Allmusic.com a lot and most albums allow you to listen to 30 second previews. Mostly, album covers are an arbitrary thing to go by, but I've got to sort through all this stuff somehow.
Semi irrelevant side note:
One of my favorite covers that happens to belong to an excellent album.
EDIT: Elphenor would dig that Neko Case album. I'm sure of it. Listen to it, mother****er! Just do this one thing for me!