Macminicolo has a great blog post about the changes in the new Mac Mini and what they mean for users. These guys know too, as their Mac Mini data center in Las Vegas hosts over 1400 Minis. They also sell off their older machines and follow the value of past Mini versions.
The 2012 Mac mini will still be very popular, keeping the prices of used Mac minis high. That machine offers the SSD/RAM upgrade options, the quad-core processor, and can also run a number of past versions of OS X.
UPDATE: And here’s the MacFixit teardown.