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Besides the stuff that's visible, like the new homescreen widget and Android that greets you the first time you boot the phone, obviously a lot of the work is under the hood. It's all about speed, as you might've heard. It's hard to accurately gauge that for a couple of reasons—we've been using a pre-release build of Froyo, so things will probably get better before the final, and we didn't install any apps, at the behest of Adobe, since we were testing Flash 10.1 on this unit—but Google promises 2x-5x faster thanks to a just in time compiler. Overall, for us, the experience seemed about the same as 2.1 on the Nexus One, though there might've a bit more smoothness to transitions and animations.
We'll update with anything else we notice that's new too.