Here wycats explains not only what's changed in the beta release, but also why it's changed, in a clear, thoughtful way that makes me feel I've been both wrong, and an asshole, about Rails 3. To wit:
One of the things that most surprised and impressed me is the Rails core team’s (and especially DHH’s) attention to detail and the experience of the beta release. For weeks, we’ve been “this close” to releasing, but the experience of starting up a new Rails app or upgrading from a Rails 2.3 app still felt too unpleasant. In this kind of situation, it’s tempting to say “it’s just a beta—people who use beta software know what they’re getting,” but that would have been a major cop-out. For many people on the leading edge, a poor beta experience will shape their perception of the product as a whole. So we waited a bit, but now we’re finally here.
Ironically, the rough time I've had working with the beta so far has (a) mostly been me having to overcome five years of Rails 0.9-2.3 muscle memory, and (b) reminded me a lot of those first couple of years, before Rails had a semi-stable API and decent documentation. So on the one hand, Rails has taken a step backward. But it also seems poised to become an even bigger, better framework than it has been.