For me in my narrow world view – Tracemonkey is a defining feature of Firefox 3.1 and will be criticial to Mozilla’s positioning versus Google Chrome (and other browsers) in the newly re-invigorated browser wars.
That said, Firefox 3.1 Alpha 2 has made some other interesting improvements, among them is the ability to drag and drop tabs between browser windows (and yes I know – Chrome does this too). There is also support for the HTML 5 <video> tag which one day will make embedding video a whole lot easier than it is now.
Clearly there is alot more work for Firefox 3.1 to come, but the latest alpha is still the next step over Alpha 1 which came out back in July.
As to when Firefox 3.1 will actually be ready for prime time – that’s an open question that Mozilla’s schedule is still leaving open.