Ok, I have been PM'ed more than once about doing a more comprehensive review on the D300 than what I have previously put here.
The other reviewers are correct about things like the Active D lighting being great, the vibrant colors, the high ISO performance, the build quality, etc.
There is no reason for me to repeat this, so I will just again say that the posters are right on the money.
There are a couple of things that I will add, however.
First, I LOVE the 51 point autofocus. I don't care about stuff like face recognition (it sort of works) since I virtually always use a single focus point anyway...
What the 51 points gives me, however, is the ability to take that single focus point and choose the exact spot I want it to focus on, which in my case is generally the eye of a critter. With 51 focus points, spread out all over the place, I can find one that allows me to put the focus point exactly where I want it and still frame the image exactly right... I can use the continuous focusing and bang, I get fantastic auto-focusing results.
Another thing I have found is that the autofocus is a lot more accurate than on the D80. For one thing, the autofocus on my 105 VR Micro was useless on the D80, it hunted all the time. With the D300, it doesn't hunt any more... and since (again) can choose my focus spot with the 51 points, I can even use autofocus in macro if I want... although mostly I don't want to.
Let me sum it up, and put my thoughts on the D300 to rest. The image quality is fantastic, the image colors are amazing, the ability to not blow out highlights or lose shadow detail is freakish, and my lenses ALL resolve better on this than any other camera I have tried. The 51 point AF system is superb.
Put simply, if you can afford one, just go buy one.
To me, the only real choice that is difficult to make comes in if you can afford to buy a D3. If you can, THEN you need to stop and consider whether you need a D3 or D300.
If you can't, then buy a D300 (or, better yet, keep your present camera and buy fantastic glass... better glass will help your pictures more than anything).