A lot of new features have been added, and the numbering convention of just adding a .1 doesn't really explain that. If I had of numbered it, I probably would have called it "6.0".
In some ways MySQL has done both themselves (and DBAs) a small injustice. While working at MySQL I met a lot of customers that tended to be conservative - they don't install first releases, but instead wait for the second release.
In the case of 5.1, just be aware that there will be quite a few more features, and with it will be more bugs. I think it's more stable than 5.0 - but you will still need to do plenty of testing.
I'm happy to see it finally released though - 3 years in the making!
But again if I had it my way, it would have been good to see a real "Point 1" release to 5.0. There were a lot of new features introduced in late 2005 that only required small addition. Changes that were large enough that the current 'no new features in a GA release' rule restricted, but not big enough to break 99% of applications.
I can only hope that 6.0 is 5.1's "point 1 release", and not just a deluge of new features 2 years late. Partitioning could be awesome if things like the "can't mix storage engine" limitation were lifted. Quickly.
I probably will be waiting at least until Percona and OurDelta update to 5.1 GA, and perhaps another month after that.
 I think Oracle causes this - having traditionally offered much stronger second editions.
 For example; I now don't have to use the SUPER privilege for triggers, but I still have no way of using SIGNAL in a stored procedure. It's a shame that for both of these the compile cache is still per-connection.