They spent a lot of time making under-the-hood changes. The biggest improvement was in taking unused plug-in memory and returning it to the OS to be used for other things. It's nice to have, yes, but if you have more than 1GB of RAM (and most people do nowadays), it really doesn't make two biscuits' worth of difference to the user.
On the GUI, the biggest thing they changed was the address bar now shows more detail about URLs that match what the user is typing -- also a nice-to-have, but I have yet to say to myself "damn I'm glad they added that, I'd never have found that website without it", so...meh.
Third doughnut when you've already had two.