For me the key difference so far has been the amazing autofocus and how much more quickly it locks into and retains focus on, for example, a bird in flight. The faster frames per second definitely makes a difference, too. The 3" LCD screen is gorgeous and easily visible out in the field in bright sunlight, and the larger size makes it even better for quickly zooming in and checking focus right on the spot.
Auto white balance seems to be much more accurate. I haven't needed to tweak + EV or - EV as much as with the D200 or D2Xs.
I haven't fiddled much with Live View yet; I'll be doing that more with macros and probably on the D3 rather than the D300. I tried out the self-cleaning thing and I'm sure that will be useful but I'm not tossing out my Giottos rocket blower, which has served me well through all my other DSLRs and has kept them pretty clean!
For me the 12 MP didn't really make a lot of difference as I've had that on the D2Xs anyway, but yes, compared to the D200, it does provide a little more latitude for cropping and retaining good resolution in an image. Someone coming from a 6 MP camera would be more likely to notice the differences, though.
Like the D200 and D2Xs the D300 has that wonderful Non-CPU Lens feature where one can adjust the metering and set parameters for those oldies-but-goodies lenses. This is a very nice feature if one has favorite old AI-S lenses from the past.
The vertical grip/external battery compartment (MB-D10) is superior to the one which is used with the D200. It fits much more securely on the camera body and feels much more integrated with the body. It is amazingly versatile,too, in that one can use an EN-EL4a battery (normally used on the D2X/s, D2H/s, D3) and get more "oomph" and even faster fps, or one can use two EN-EL3e batteries (the ones used with the D80, D200 and D300) and be able to be out in the field shooting away for a long time without needing to swap batteries. If one so desires, the battery grip also works with various alkaline, Ni-MH and Ni-CAD batteries, too. Take your pick!
Speaking of batteries, the D300 seems to be better on battery life than the D200. That is a relief so that then one doesn't need to cram pockets full of batteries to go out for a day's shooting.
If I were starting out fresh today and making a decision about which camera to buy I would not hesitate -- I would go right to the D300.