I was actually very pessimistic about the 4.2.1 update, and didn't upgrade until the untethered jailbreak came out. I am glad I updated.
I would show off the way you can easily switch apps using the multitasking bar. I would show off the very intelligent idea to always have media controls in that multitasking bar, which controls the last used media app. I really like this feature.
After that though, I would have to show off a bunch of the jailbreak features I have added. The only other things are some of the apps.