The README in libwiiu explains this. You can run userspace code, which is enough for most homebrew, but doesn't let you poke at other apps.
Just out of curiosity, not wanting to take the thread off topic, is there anything we can do in userspace that cannot be done in the browser as is with web technologies? Currently not in a place where I can look it up myself. Mostly curious at how much Nintendo optimized WebKit since they allow HTML5/JavaScript/ImpactJS game releases in the eShop.
Has anyone tried running the asm/emscripten compiled emulators out there (Retroarch, snes9x, vba, etc) to see how they run on the Wii U browser?