Like just about every crazy purchase, you must have your excuses. (I won't say reasons, since they are indeed excuses.) There is are quite a few reasons why I bought this particular model, and I am quite sure many people, even Canon users can come up with them too.
1. Access to more lenses - the D90 comes with the AF motor needed to drive many older design lenses - using them could save you a fair bit of money over the equivalent newer ones. If you buy plenty of lenses, this pays for itself bloody fast.
2. Wireless flash system - the D90 comes with the ability to use wireless slave flashes - no need to lug around an extra flash like on the D60 to do the same commanding. (the extra flash can now be a wireless slave too!)
3. The ability to keep ADR on Auto at all times - this might not matter to most people, but this is a lifesaver on many occasions for a newbie like me. The overhead of ADR on the D60 is just way too high to bother.
4. Live view - no need to go blind when taking those high angle shots - you can now see what you're doing.
5. More buttons - the nikon D90 has plenty of buttons so you can change just about about setting that matters to you. No more weighing pros and cons of assigning the Fn button - instead you'll be busy looking for something to assign to the Fn button instead.
6. The screen - the 3 inch screen gives dSLRs what would probably be the biggest gripe of upgraders from p+s a big kick out the door. The screen is brilliant, sharp and shows you so much more. It's less a matter size than it is resolution.
7. Faster autofocus - it ain't the D300, but the AF system is faster than the D60, it is however more confusing to use. Coupled with a fast focusing lens - the camera feels crazy fast compared to the D60's occasional hiccup. (Of course, while it is indeed more confusing - it's faster - and actually means you can just point and shoot - less recomposing required. )
8. Movie mode - not really a reason for me, but it does mean i don't have to bring a p+s around to take the occasional video - i can just go all over the place with merely my D90 and 18-200 VR.
9. Better battery life - unlike the D60, the D90 does have an official battery grip for it, in addition to having a longer battery life to begin with.
10. Sensor - the sensor is ISO 200 to 3200, more sensitive than the D60. This essentially means better lowlight performance, not to mention this sensor should be highly similar to the D300 - giving us plenty of bang for our buck.