The Aero Peek feature has been received quite positively, as it really can be quite useful when working with lots and lots of windows. It reminds me a lot of Expose - not in how it works, but in how useful it is. When I first heard about Expose, my reaction was "meh". It felt like useless eye candy for Apple Store owners. However, as soon as you started using Expose, it became clear it's a massively useful feature, and I still consider it one of the best additions to window management since, like, forever. Aero Peek falls in the same category: looks gimmicky at first, but once you're used to it, you don't want to live without it. In the RC a much-requested addition has been made: Aero Peek can now also be triggered by the alt+tab switcher.
Another addition that really makes me happy (and I'm sure lots of other users as well), is the ability to drag/drop a file on a task bar item and have it open the dropped file in the program you dropped it on. For instance, if you have Notepad pinned to your taskbar, and you drop a html file on it, it will open that file in Notepad. This behaviour is not default, however; you need to press shift as a modifier. Still, a very welcome addition, as it allows for an easy way to open fiels with non-default applications.
In earlier Windows versions, the Win+e command launched an Explorer window located at My Computer. In the Windows 7 beta, this often-used command started in the Libraries view instead, which didn't make an awful lot of sense for many people. In the RC, the behaviour is back to default: Win+E opens an Explorer window at My Computer(or Computer as it's called these days).
A small but funny one: the on-screen keyboard now supports multitouch. In previous builds, the on-screen keyboard wasn't multitouch aware, which kind of made typing rather weird. Now you can type with them as if it were a real keyboard (I'm not serious, of course; typing on a touch screen will always be cumbersome).
There's more in the list, but they reiterate it's just a selection.