Habits and hardware are definitely the big ones to overcome. I still remember how absolutely lost I felt the first couple times I tried installing slackware in the 90s. I could install/set up windows in my sleep. But then slackware dropped to an unfamiliar command prompt, I can't dir, there isn't even a C drive, and now I'm expected to configure something called xfree86. Luckily I wasn't told to use vi or I'd be stuck there to this day.
New users aren't thrown into the deep end quite like that anymore, but it's still a big change for a windows power user. So much of what you learned is not applicable or just the wrong way to do things. Mac users and Windows non-power-users seem to have a much easier time accepting the changes.
It's definitely not for everyone (is any OS?) but it's been 'ready' as a desktop OS for me since Mandrake 8 in ~2001. That's about when I ditched windows 2000 and haven't looked back.