For your situation (OP), there is not that much upside to making the change. I clung to the "1" until Apple finally went 1080p because I had had a 1080p set for a very long time at that point. I'm fairly happy with the change but only for that reason.
Airplay? Almost never use it myself
Netflix? Hulu? Never use it (but also had it as apps in the TV itself while I still had the "1")
Maybe these features are important to you? But then you have Boxee and Playstation to cover those latter ones.
I still think the "1" has the better UI (your own "movies" are in "movies" not "computers", your own "tv shows" are in "tv shows" not "computers" and so on). In fact, I'd most like to see Apple release the last incarnation of the "1"s UI as a new "front row" and then I would probably just get a Mac Mini to replace my "3".
Yours is also capable of several things the new one can't do. For instance, I really miss sync. Store a good number of movies on the device and turn off that computer. Or take it on vacation with you but leave the master Mac at home.
You can also apply some hacks so that you can attach at least 2TB of local storage for syncing.
If you wanted 1080p, you can add a hardware hack to get there too.
Yours is also the only one on which you could pop your own popcorn or fry an egg (the spare hotplate/space heater advantage is gone with the "3").
In my perfect
TV dream, the last UI for the "1" would be fully brought to the more modern hardware of the "4". For those that want to sync, the handicapped USB port would be turned into a fully functional (hook up whatever local storage you want for syncing), an audio stereo out jack would be added for "zone 2" receiver purposes, it would support 24fps and 60fps along with 30fps native, and it would fully match BD in terms of video and audio codec playback so that BD has no tangible advantages over
TV. There would be an app store so that other companies could bring the features to it that Apple seems too distracted- or in no hurry- to bring themselves.
If you are going to stick with that 720p HDTV for several more years but you see something you want in the newer
TV, consider a used "2" as it is just as capable as the "3" for your TV and it can be hacked for some added functionality if you want more than Apple gives you. Used "2"s can be had for pretty cheap. Maybe pair a used "2" with your "1" for the best of both worlds?