Price. Don't waste $1000 on a screen, look for bargains.
In saying that, things to look for:
Good color performance and clarity - Which colours look the best, if you see a bunch all together showing the same demo dvd, screensaver or desktop, look for one that looks good!
Sharp text - Blurry text or hard to read text isn't good at all, make sure you get a demo of looking at text based websites, etc.
Design/appearance - Is it aesthetically pleasing?
Video input - How does it all plug in? Some have new digital cables only, which only connect to newer motherboards, meaning you need an adapter. Make sure it'll all work with your machine before you buy.