Having owned an iPad 2 for a while now, I would buy an iPad 3 if it had an increased (i.e quadrupled) resolution. I hesitate to call it a retina display at quad-iPad res, because it is lower DPI than an iPhone 4's retina display, but it's close enough. The main reason I bought the iPad 2 , is as a replacement for newspaper & magazine subscriptions (I tend to collect too much paper) and books and for lightwieight web access (i.e as a reader). As a magazine reader it is too low res for me - I am constantly zooming in and out - trying to see the big picture, but then zooming in for text, or switching to text mode and missing out on the layout and artwork. As a pure book reader it is better, but I still wouldn't mind sharper text. There is a big fat bezel on this thing, so I wouldn't mind a 12" screen in the same form factor, but I guess they have to save something for the future (maybe when OLED comes down in price and having a 12" screen would not add to the thickness / weight / power draw?)
Quad-core is a no-brainer improvement too, but the feature I pray for most is quad XGA.