The reason 120Hz HDTV's came about was to reduce motion and blurring in LCD TVs. However, the technology isn't widely available to consumers yet to support this feature and in the end, it makes the picture look cheesy and is a disgrace to the very picture it is trying to reproduce.

*Note: This post on the negative sides of 120Hz only effects movies, or content that is shot at 24fps.

Technical
Hertz (Hz) is a measure of frequency or cycles per second. See Hertz for more information. So in this article fps and Hz will be used together because they both operate at cycles per second.

Most film is shot at 24fps but all TV's display at 30 or 60Hz. So the solution was to do what is called a 3:2 pull down, more information can be found in this article: DVD Benchmark Part 5 - Progressive Scan DVD. Basically it takes 4 frames and converts them to 10 interleaved frames.

This wasn't really a problem until HDTV's came out and went from 480i to 720p and 1080p which introduced progressive scan.

With progressive scan, the same 3:2 pulldown takes place but every two frames of the 10 is put back together and the third is thrown out. But when doing the progressive scan you actually see those few frames getting dropped this is called jutter.
At first manufactures decide to just increase the display of the 3:2 pull down from 30Hz to 60Hz. Ideally though the perfect solution would be to have the film content displayed at 72hz (every frame shown 3 times, 24x3=72hz). This would eliminate the jutters because no frames are dropped.

The wise to be manufactures decided that instead they would just show you the 3:2 pull down four as fast so you wouldn't see the jutter because now each frame is being shown at 1/120th of a second. Its too quick for your eyes to see the juttering. One problem, now it has a rather weird side effect, in my terms "It looks like crap".

Explanation
If you are here in the United States you are used to a format called NTSC which displays video at 30fps. Over in Europe there standard of film is 25fps so their tv format called PAL displays at 50fps. So, whenever you see a show that was filmed over in Europe it looks odd because of the conversion between frame rates. 120Hz TVs look similar to that, except worse.

If you have ever looked a 120hz display in a store, you will notice that when you see a movie being played that it looks kind of like the behind the scenes footage of the action scene. This is because of the 120hz conversions that are going on.

Solution
With all of that negative said, 120hz makes gaming a dream. It enhances and looks better then ever. So my solution is to have multiple refresh rates based on the content that it is displaying.

If your TV is displaying a HD movie at 24fps, display at 72Hz. If you are gaming, 120hz. Or manufactures could get on board and support true 5:5 pulldown and it would solve everybody's problems.

Additionally, 120Hz was a solution to a problem within LCD displays, there is almost no benefit in have a 120Hz refresh rate in a Plasma TV. Do not be fooled by marketing into getting something that is considerably more expensive because they say it looks better. You won't be able to see the difference.

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