TVs, like speakers, come in all price ranges and quality tiers. There are many nice options out now and they are improving with every model. You can look at specifications until your head hurts, but here are a few things I look for when comparing plasma sets that may help.
1) Contrast – Some sets are better than others at producing a black level. A darker black will generally produce more contrast, or depth of image. High contrast is great with a bight clear image such as a football or baseball game in the sunshine for a crisp image. The down side is that you may have a harder time seeing detail in darker scenes in movies, TV shows, etc. TV’s with a higher contrast will want to make the gray and dark images more of a single black color. TV’s with a more varied grayscale will allow you for better vision in the darker scenes, by showing light gray, dark gray, black, etcetera. Generally, these sets have a greater variance in their grayscale but not as dark of a black. Thus, not as much contrast giving you a softer picture. Some of this can be adjusted in the brightness and contrast, but each brand will tend to have its set of characteristics. The question becomes: do you personally like more contrast or better grayscale?
2) Color – True to life color is tough for plasma. They are very vibrant, but somewhat exaggerated colors. The scale of color will make a difference from TV to TV. If you compare an image from one set to the next, take a look at the shades of a specific color. Green can be a particularly tough color to reproduce well for plasma. If you can see an image with a trees or plants, take a look at the different shades of green. Some sets at better than others at making the ferns a different shade of green from the douglas fir and the maple trees for example. The sets that do not handle color as well will want to make these all one shade of green.
Since this is already more wordy than I had planned, here is the ‘Cliffs Notes’ version of the rest. These are more things to compare brands when shopping.
3) Motion – Look for artifacts (blocks in the image) with fast camera movement or fast object movement such as a basketball game.
4) Up-conversion – How well does it take an analog signal and display it at the TV’s natural display resolution.
5) Shimmer – for lack of a better word. Does the image artifact, flatten, or shimmer when you see reflection such as spot lights shining on a hockey rink or the sun setting on the ocean.
Don’t let the fact that I references sports on a few occasions give you the impression that I am a sport fan. 
….ok, I maybe am. 
I am not the ultimate video critic, but some of these things may help you in selecting your plasma set.
-Korey