All previous versions were just release candidates, whereas this release is the real deal.
I used the Nightly Tester Tools to override my extension versions so that they will work (somewhat) properly with Firefox 3.0.
Any extension that does not have a secured update location was removed from my laptop, on purpose.
As more people adopt 3.0, I suspect more extension authors will start using secure update locations, because, by default, 3.0 makes it difficult to use non-secure updates.
All in all, this version seems to use much less RAM than FireFox 2.0 with general use.