What are the qualities of a top end paint versus a low end paint?
I have always been pleased with Behr paints from the Home Depot, though I know they are a bit pricey.  

From my experience less expensive paints tend to have the following in common (and I'm sure this is by no means comprehensive)


less covering ability ... the pigments used don't seem to be able to provide as good a barrier to bleed through from what's on the wall now
less coverage ... probably related to the first point, but they also don't seem to get the same amount of square footage per gallon
more spatter/drips, the paint itself seems thinner leading to more cleanup
less selection of colours
less durable, the less expensive semi glosses that I've used don't seem to retain their finish as long leading to repainting earlier.
more marking, again related to the finish, the walls seem to mark up much easier


Again this is only based on my experiences, Sears used to have a good quality paint but lately I've bought Behr.  CIL is probably good in most cases as is Benjamin Moore.  I've used a number of bargain brands and don't really recommend any of them, and the last time I used a gallon of Walmart house brand was the last time I used a gallon from Walmart :-P

Hope this (mostly opnion) helps.