Windows recovery tools take up 5 GB and Windows RT, Office and built in apps take up another 8 GB.
I can't help but feel that when so much space is being used by built-in software, the whole device must feel sluggish and bloated. Judging from some reports, this is an accurate assumption.
Interestingly, 32 GB iPads have 28 GB of usable space. They also don't feel sluggish. Perhaps the slimness of iOS may account for at least part of this?
Considering Microsoft is a software company, this turn of events surprises me. I'd expect the software coming out of Microsoft to be great, and the hardware to be questionable. But it's the other way around.