Some aspects about LCDs are subjective matters of taste, like wide-screen vs 4:3 aspect. But there are quantitative differences between panels as well, so a side-by-side comparison is always useful.
Make sure you're getting an LCD with a DVI input. It just doesn't make much sense to drive a digital display with an analog signal.
Response time and panel technology will dictate whether the panel has a "ghosting" effect after fast changes.
The backlight should be bright and even.
The LCD should obviously be free of stuck or dead pixels, and the anti-glare coating shouldn't cause any weird artifacts (like moire patterns). Some panels are available with glossy coatings as well -- some people like them.
Color rendition varies considerably across panels. Samsung LCDs tend to run warm, which I like. LG's tend to run cold (colorwise).
The larger the panel, the hotter it'll run (mostly due to the backlight and inverter). Avoid LCDs with noisy inverters.
Go with at least 1920x1200 resolution if you get a >20" display unless you have really bad eyesight, in which case a lower-res/larger-font might be appropriate.
Different viewing angles, both veritcal and horizontal, to consider as well. And consider the available inputs if you plan to hook different devices up.
Like I said, the Dell 24" is superb in all of these areas. I've heard that the Dell 30" is even better, but that's a BIG monitor and requires a lot of videocard horsepower.
