@Bob, well, distrowatch is a good place to start. I actually stumbled upon a website that does a lot of linux distro reviews whilst looking for a grub tutorial, and that's been a place that I've found out about a few new distros.
I think it comes down to what you'll want to do, and how easy it'll be for you to configure it. For example, a security distro such as backtrack is full of network testing utilities, which are all configured in a way to work well with eachother. Could a user set it up? Yes. Better than the bt folks did? Maybe. But why bother when it's already set up?
That was the main reason I stuck with mint for so long, it was ubuntu but with all the things I was planning to add anyways. Those are some of the things I look at. And I tend to prefer Gnome 2 to other desktop environments, so that's always a plus for me.