Hi there,
I have the 4.2 GM on my iPod Touch 2G and I can tell you that it's not a major update for us 2G users. Here are some things I noticed about the 4.2 OS. Note, these could change by the time the official update is released:
1. Webpage search is missing
2. Starting the jiggle mode and ending it seems to be a tiny, tiny, tiny bit more delayed then 4.1. Not a huge delay, maybe about half a second, but it's not a big deal. (It may be if you're picky!

)
3. I've noticed a tiny performance boost on my end, maybe about 5 to 10%?
4. In the App Store, it will say whether a game is Game Center enabled (it's kinda built into the app information, if you know what I mean).
5. Some more restrictions added.
6. Voice Memos icon changed, looks better than the previous icon, in my opinion.
7. Two other Note Pad fonts added, both better than the Marker Felt we are all unforunately accustomed to.
8. Opening folders is a tad bit more snappy.
9. And some other minor things I saw, like some icons moved around in the photos app, Birthday icon changed in the Calendar app, etc.
As you can see, those are all minor stuff. I also want to mention that I couldn't seem to find AirPlay anywhere.
Thanks, and I hope I helped.