There are lots of Twitter apps, so that's not a big thing. The Facebook web page works pretty well in IE10 on it, but yeah an app would be nice... But the integration in the platform sort of takes care of it... Sort of.
Hoping the Music/Video apps are coming soon... Can recommend Xbox Music; it's got more music than Spotify and looks AWESOME.
I use ngreader (Next Generation Reader) to read my Google Reader feeds. It's amust on any Windows platform. Also Angry Birds Space and Star Wars is awesome! Would also like to recommend Adera; almost like Myst for those that remember... :)
I know its wishful thinking at this point but I miss a lot of Google apps like Google Drive, Google Talk, and Google Chrome. Google+ & Google Reader native apps would be nice too I know there is no Windows desktop app for either of those but the Apps they have out for Android would be perfect on the surface too. Google+ works pretty well through IE10 but Reader is pretty unusable.
I have IM+ to connect to Google Talk at the moment but its not an ideal or efficient interface.
Google Drive I have been using through the web interface in IE10. It seems pretty sluggish. There are some odd quirks to when editing doc with the onscreen keyboard. I know I could start using SkyDrive more for this type of functionality but I am heavily invested already in Drive and have other collaborators that I share docs with through GDrive that would be challenging to integrate into SkyDrive as well.
When it comes to Google Chrome, I really miss the book mark sync and all the other Chrome bells and whistles. I've attempted to use Xmarks to get my chrome bookmarks synced up from my desktop machine. Again, it works OK but not great. Nothing beats the real thing! We know they have a pretty good engine for Chrome that already runs on ARM for Android so I am crossing my fingers that someone at Google is working on porting that over to Windows RT!
Outside of the Googleverse, I would like to see official Dropbox, Pandora, Facebook & Twitter apps.