Plasma TVs are still seriously worth considering. They still tend to beat LCDs on black levels in particular.
Aspects of the TV to consider:
- Black levels (a good plasma beats a good LCD here)
- Colour accuracy (not just vibrance of colours. There's a difference! Again, a trend tends to be that plasmas are a little more accurate but LCDs look a bit more vibrant)
- Brightness (TV manufacturers compete on brightness in a big way, because it makes their TV stand out more in a big bright store. But when you get it home that brightness can be very over-the-top in "normal" lighting conditions)
- Viewing angles (most TVs quote near-180 degree viewing angles and it's often BS. And it's not just about whether you can see a picture at extreme angles, there's also the question of whether the colours stay the same at relatively small angles)
- Response time. This is the same issue you get with LCD monitors. Basically, does fast motion (a ball moving across the screen, for e.g.) leave a blur behind it that wasn't in the original image? Screens with high response time tend to suck for gaming (consoles) and fast-action films/sports etc. They used to say it was no good watching golf on LCD TVs, because on old LCDs the golf ball would vanish once it started moving.
- Quality of the image processors. Much of the content you view on the TV, whether it's a '720p' or '1080p' TV, will not be in the TV's native resolution. Therefore it's very important that the TV can scale the resolution well so that a non-native-res source looks as good as it can.
- Resolution. I sort of sympathise with those who say this is the single least important aspect of the TV. Of course, it depends on how big a TV you're getting and how far away you are sitting from it. Unfortunately the resolution is probably the detail that gets the most attention from buyers because it's the one with a simple number attached, and bigger=bettar! Also because the big LCD manufacturers hyped it up a lot in the past years (Sony in particular) because LCDs tended to have an advantage in resolution in the past. Nowadays plasmas can be either 720p or 1080p without any issues.
- '24p'. This is the feature that allows blu-ray video to be played at its native speed, without any skipping of frames/juddering motion. A surprisingly large number of TVs don't have this. Perhaps a 720p 24p TV would be nicer to watch blu ray on than a 1080p TV without it.
Those are all the important features I can think of right now. There might be others too.
As for plasma vs. LCD: Plasma tends to have more accurate colour, darker blacks, somewhat less brightness, less vivid colour (but, as I said, more accurate), and less issues with response time (though they get a different kind of response-time-style issue which I am not that well educated on). Better viewing angles too I think. But these are generalisations. It is all down to the model in question.
Look up reviews of the TV you are considering! Definitely advisable. Specs really don't tell you anything, you need to see one for yourself and/or read experts' (or other users') opinions.
If you're interested in a Plasma TV, the Pioneer Kuro range are the dogs bollocks while Panasonic make 90%-as-good-but-60%-as-expensive alternatives.
Here are a couple of useful forums you might want to browse:
http://www.avforums.com/forums/index.php (a UK forum I've lurked in a lot)
http://www.avsforum.com/avs-vb/ (American forum, same sort of discussions but obviously a bit more relevant to the American market)