It's a tablet that's has a different functionality set than other tablets, not a PC. What I mean is that it can run Flash, which the iPad can not, and you can connect USB devices to it, which the iPad and most Android tablets can not, but there will come a time in your Surface RT-owning life that you will want to install something that works on Windows 8, and it won't work because it's not made. Maybe it's a simple utility or a plugin like the one that lets you use Google Hangouts. Also, the app store is very young and the availability of apps is much worse than even Windows Phone 8 at this point. Despite over 100,000 apps in the store, off the top of my head I can't think of a dozen truly top tier apps that are must-haves on the platform, and that likely won't change by the time the next Surface is released.
Overall, I'm satisfied with my decision to go RT as it suits 90% or more of my "PC"-like needs, but I will definitely be getting a Surface Pro 2 as my next device for that other 10%.