Well considering that Windows 8 is a REAL OS, as opposed to iOS / Android, I don't find it to be that big of a problem.
20GB is plenty for a tablet user; they aren't going to install 20GB games. On this kind of tablet, you're really just going to install a few programs, some music, and maybe some video.
The Surface Pro is targeted towards ultrabook users that mostly do some word docs and internet surfing.
This isn't an entertainment tablet. There's the iPad for that.