As many have said, like them I would purchase something purely because it included my favourite characters and because I want to see them evolve etc. 

However, if I'm browsing and I have some money to spare, the cover is basically what seals the deal for me.  I mean, if it has a cover by Marko Djurdjevic, there's a complete gurantee I'll pick it up.  Also, any cover by Mike Choi usually, but I tend to buy everything he works on so that's okay.