OK, you could say I'm a cover lover. I always keep my library up to date with the clearest digital copies of album covers and whatnot, even if the tags aren't correct. Also, when I play music, the largest part of my foobar setup is the artwork. I very recently jumped into physical recordings (yes, I'm young) because of my love of the cover art.
A good cover to me is one that accurately conveys the style, sound, and possibly the overarching theme of the album on the cover. The best albums don't do this exactly, but rather add onto that experience, almost making the release somewhat a whole, cohesive package. In other words, you are missing part of that album by not having the cover in your hands and looking it over while listening to the album.
There are plenty of examples I could pull, but I'll choose this one because I like it.
Wolves in the Throne Room - Two Hunters
(on a side note, this album just slipped out of my reach. The last vinyl copy sold right before I could buy it off Amazon. Looks like I'll have to get it off of the scalpers on ebay. If you have a vinyl copy, PM me)