full disclosure: I'm a 10.04 user and plan to stay that way until the next LTS.
If you tell a "regular Joe" user that by clicking a dropdown you can change how the operating system fundamentally works, they'd probably think you were lying--the thought is that alien to them. There's a slim to none chance they'd discover that option on their own (not in small part because they wouldn't even know the option existed), and an equally slim chance that they'd know how to search for it.
To you and me, "Unity" is the "well, duh" search term. To regular people, it's tech gibberish.
So again it all comes down to the average user--they don't like change, they don't keep up with Ubuntu news, and they aren't interested in fiddling with configuration options to have a usable desktop. I don't think that you can have both fundamentals-shattering releases and also a ppa-based/milestone-release distro at the same time.