I would love to see some AAA Xbox titles get some ports:
Borderlands
Halo
As far as apps:
Youtube
Chrome
Remote Desktop (Does this already exist for RT?)
Music recording software (I would go nuts for audacity, reaper or any other free daw)
Eclispe/Visual Studio - could you imagine being able to program for ARM on an ARM device? the education benefits would be huge!
Guitar Pro (Guitar Tab literature software)
Text books - I like how Apple has textbooks - I never used it because it's too cramped on my iPad screen but on a 10.6 inch 16:9 screen it would just scream.
Edit: Would love WebEx for work, too!
Everything else I can think of can be done through the browser like you would on a desktop or x86 computer (like facebook etc) - apparently Facebook games work well on the Surface.