I'm on a Captivate right now. And I there are 4 main things that I want a future phone to have.
Also, a quick disclaimer. I'm talking about the Galaxy Nexus on Verizon, not the International version.
1. 720 Screen - This will be the new standard among Android and WinPho7 in the coming years. I honestly don't see a need to go to a high resolution on a < 5" screen. For web browsing, having those extra pixels makes a huge difference.
2. LTE - If I'm going to be committed to a phone for the next 2 years, then it would be a shame to miss out on LTE. The speeds are dramatically faster than 3g. Even if you don't have LTE in your area right now, you probably will by 2014 when you can upgrade again.
3. At least Dual - Core. Both have pretty great processors, and will get the job done. The TI chip in the Nexus is currently the only chip that allow's LTE, so it's a necessary (if minor) drawback.
4. NFC - Much like LTE, it isn't quite ready for prime time yet, but I think it will be a big deal in the next two years.
So the 2 big things the GSII is missing are LTE and a 720 screen.