Even though it's sponsored by Sharp, this video is a good primer on what to look for in an HDTV.
Main points: LCDs with matte screen are best for rooms with lots of light where control of lighting environment isn't possible.
Buy the biggest screen that you can afford. This can't be overstated. If you think you only need a 42" TV, you are wrong. It's not going to be big enough when you get it home. Always get bigger than you think you need as long as you can afford it.
Always buy 1080P. Unless you can't afford it, always buy 1080P. There's no reason not to and these days there isn't much difference in price between 1080P and 720P sets. Plus most sets above 40 inches in size are 1080P anyhow these days.
If you buy LCD and can afford it, get one with 120 frames per second or 120 hz option. These screens will be better for motion than normal LCD screens that have slower latency. In other words, a 120hz LCD screen will have less motion blur than a regular 60hz LCD screen.
Buy a sound system that matches your TV. Actually, here I'd say your sound system (including receiver, speakers and subwoofer) should be twice the price of your TV or more. Sound is actually 80% of the experience of watching a movie for instance. You can close your eyes while watching a movie and imagine what it looks like in your head, you can't do the same nearly as effectively with plugging your ears. So if you buy that 60" screen, don't skimp out on sound with one of those home theater in a box things.
Oh and for large screens (50" and up) rear projection is still the sweetest price/performance choice, although they are becoming more scarce, particularly in the 50" size. If you do go rear projection, don't get a traditional DLP, make sure it's either LED lit (like most Samsungs) or laser lit (like the upcoming Mitsubishis). Traditional DLPs using color wheel are not nearly as good with motion as the newer lighting systems and can cause certain artifacts like rainbowing. Avoid them if at all possible.