I’ve been working with Lightroom 4 Public Beta quite a bit (OK, a bunch), and I can tell you exactly why I think this there is going to be a tidal wave of people upgrading to Lightroom 4 when it comes out, and it’s the most basic reason of all: Your photos look better processed in Lightroom 4. Period.
At the end of the day, that’s what we all want. We just want our photos to look better, and if somebody comes up with something that can make our photos look significantly better, we’re going to be all over it. Lightroom 4 does exactly that.
The improvements in Lightroom’s Development module are so significant, and so much better than what we’ve ever had before, that I think you’ll be hard-pressed to find most anyone still using Lightroom 3 in just a few months from now. In fact, if they didn’t add another feature, it would still be worth the upgrade just to get better looking images. If you haven’t downloaded the beta, here’s the link—see for yourself. :)