This cover from Velluminous Press in the U.K. may appear simple, but notice that there is actually quite a bit of detail. The background is sort of like an old worn parchment and that big detailed eye, with lids formed from the background material, instantly captures your attention. These elements combined with the odd title, give you no idea about the story within, but you just can't help but be curious about it.
This cover from Breakneck Books Publishing has alot going on: A forboding castle set with appropriate shadowing when compared to the dusky sky above, an armored hand weilding a vortex of power, and the dark knight on the back. Notice how the cover text is all positioned correctly and the different text styles all compliment one another. Everything says, "Professional." If you're thinking, yeah they paid big bucks for that cover. You'll be surprised to know that the owner of Breakneck designed these covers personally, albeit not without some artistic background.