Although I suppose it all comes down to what you define a service pack to be, Windows 8.1 isn't a service pack. They're going to completely refresh the OS and flatten the entire Windows directory. There's changes everywhere from Windows Explorer to IE to compiler improvements to new syscalls - at least going by the leaked build.
It might "feel" like a service pack, but it really isn't. It's an entirely new OS whose UI is pretty much the same with presumably a transparent upgrade path.
Wow, really? It's surprising that MS would make all of those under-the-hood moves in a minor version update.