Not to be pedantic or anything, but "full OS" is not the real problem, the problem is trying to work a traditional GUI directly into tablet form. The concept, in the past, has been to build a tablet onto which you can install all your favorite applications and use them as you might from your desk, albeit, perhaps with a twig instead of a mouse.
Apple simply designed iOS to take advantage of what the iPhone had to offer and gradually expanded it. iOS is still a fully functional OS, Apple just works at keeping your fingers out of its innards or using it to the fullest extent that you might be able. Many of us are hoping that the leash will get longer and looser over time (sooner would be better), but for now, I think Apple wants to reach out to the most averagest to provide them with a stable, secure, simple replacement for the big overpowered computer that was designed for someone other than them.