A few thoughts from a newbie Ipad2 owner, but a somewhat long-time Apple user on other devices...
I jailbroke my daughter's iPod touch 2 days after I bought it for her. Having done that, and gotten SSH access to the root shell, I can definitely say it's (basically) a full UNIX implementation, with (as far as I can tell) the same environment as Mac OS X, for example launchd.
The "operating system" is UNIX - and stands on it's own merits for obvious (to me) reasons.
The "user interface" is pure "i" - iPod/iPhone/iPad. There is no way to get to the underlying operating system from the user interface.
And most/all "apps" run in the "user interface" environment. Well, for all intents and purposes, we can use that to describe the "i" behavior
You can justify this as a way to keep viruses away, or use it as an example of Apple's proprietary-ness. Either way, it is what it is.
I have definitely found some things strange, as in "home sharing" doesn't even show up on the iPad unless a computer is on the Wi-Fi network that is home sharing it's iTunes library. If there isn't one around, it's not even a menu option. I understand it from the "dumb luser" perspective, as in why offer the clueless user something that won't work. But using it the first time, it's hard to understand where something might be, and if it's not enabled or working you can't even find it.
The moral of this post: iOS itself is a decent operating system. The user interface is what owners are faced with. The limitations of the user interface are there for a reason, whatever that might be.
Apple is crazy like a fox. There is a LOT of potential in these devices, and they keep people interested by slowly, incrementally, giving more and more versatility to owners. On the one hand, kudos to them for doing it for free with the next update. On the other, well, the developer/tinkerer in me hates the closed environment model.
It stifles innovation. But not for Apple