if there was no difference between 60hz and 120hz then we wouldnt have so many hardcore gamers clinging on to their old crt's.
To be fair, the difference is night and day between 120hz vs 60hz on a CRT, and on an LCD and shouldn't realistically be mentioned.
CRT's flash the image on screen, at 60hz its very flickery and can induce headaches, eye strain and be generally awful. At 120hz the "flickering" is up to a level your eyes can't perceive it so the eye strain and so on is reduced significantly. There isn't ghosting on a 60hz crt, however the increased framerate helps.
On LCD's they don't flicker, they don't induce headaches and moving to 120hz really barely changes that at all. Ghosting is something thats present on a LCD though, and the increased framerate largely increases the responsiveness of the panel.
With overdrive response and the like you have screens trying to compensate for ghosting, however on a 60hz screen this doesn't always give it time to where its going before the next frame goes up on screen, the 120hz gives it 60 extra frames in which to "compensate" for ghosting. If you look on one of the better uk tft review sites they highlight the difference between 60 and 120hz mode, which is largely down to lack of ghosting AND lack of overdrive response overcompensation, which finally helps bring us towards "almost" ghost free LCD's. Then on top of that the 120hz situation helps with tearing a heck of a lot and many other issues.
The last big improvement is simply the ability to show 120 different frames on screen IF you have a system capable of providing 120hz in games, which is actually one of the smallest improvements of a 120hz screen.