I'm somewhat looking for a new monitor too. My 9 year old HP is working fine, but I think I just want to upgrade..
A few things I think are important to check on a monitor:
format (16:9 or 16:10)
panel type (TN, VA, or IPS)
screen type (glossy or matte)
connector types (HDMI, DVI, Displayport, Mini Displayport) -- I think for Mac you might need to check this especially
16:10 (think 1920x1200) are really nice if you intend on working with documents, multimedia (audio or video) work, or website viewing as it provides more vertical screen space than the normal 16:9 ratio screens (think 1920x1080).
With panel types all IPS are not the same -- there are different ones and the kind you get may have a different ability to represent colors. For instance, the cheap eIPS are generally 6 bit + FRC to mimimc 8 bit color.
The expensive ones do full, 10 bit color. But you need a video card that can output 10 bit color. I don't think the off the shelf Geforce or Radeon style cards do this in 2d mode -- rather you might need one of the workstation type cards.
My recommendation would be to go to an electronic store and get an idea of screen types. Of course the ones on display generally aren't calibrated and probably have been adjusted by numerous other customers. But you can still get a feel for panel type, screen surface (glossy or matte) type, and features.
I've discovered I loathe Matte finish screens. The thick grain looks horrible on print/website viewing and is very tiring to my eyes. So I go for glossy screens, despite them reflecting like a mirror. Plus glossy generally punches color a bit (cheap trick).
In any case.. there are a few things to consider.