I didn't buy the Arcade Fire's album for a long time because it only came in a crappy cardboard case, but eventually gave in. That kind of casing does piss me off though.
This happened to me too, and I agree that CD casing is one area where homogeny is the best route.
I have to admit the artwork does have an effect on me, though it's mostly subconscious and only changes the amount of time it takes me to buy an album, not whether or not I actually will purchase it at all.
Really, great artwork enhances the experience more than bad artwork detracts from it.
In the Court of the Crimson King, Odelay, and of course
Dark Side of the Moon are already stupendous musical works, and the brilliance of their covers/liner notes only helps.
Where my ire rises is when I encounter disastrous lyrical arrangement in the liner notes (i.e.
Siamese Dream), but that's still a small concern.
Some truly great covers, in my opinion, are those of
Unknown Pleasures, In the Aeroplane Over the Sea, Low, Spiderland, Daydream Nation, and
F#A#Infinity.