I guess it really depends what kind of user you will be but in my case - In addition to access to the internet 24/7 w/ the best mobile browser available, using Google Maps to find anything that is around you, and push email without relying on finding wifi access point - I also use Facebook, Weather Bug, ebay, AOL or Pandora Radio (free music of all kinds streamed!), Google and google earth, Now Playing (theater app), ZenBe (a todo list manager that syncs flawlessly online), yelp or urban spoon (to find places to eat), Save Benjis (on the go price comparison for just about anything), iTV (TV listings), and anything else that peaks your interest in the app store. Of course all these apps can be used with the iPod Touch and Wifi but from my experience it can be hassle to always rely on WiFi.
Another negative is that the iPod touch doesn't have an external speaker (unless the new one does?).
I feel that the internet connectivity is the main thing you will miss with the iPod Touch. Once you get used to having a constant data connection it's hard to go back...