Background:
I ordered my D300 the second it was announced.  While I wasn't seeking to replace my D200, I did want a second camera body.  I was thinking about another D200, but when the D300 was announced for about the same price... why not?  I've shot for three days with the D300 now, including a lot of side-by-sides with the D200.  Below are my observations to date.  Note that I have not attempted to provide detail on every aspect of the D300 compared to the D200.  Rather, I have concentrated on those items likely to be of most relevance.

Summary:
If you already have a D200 and don't need another body, there really isn't a compelling advantage to upgrade to the D300.  However, if purchasing new, I'd get the D300 over the D200 without hesitation.  If you can afford the upgrade, go for it - you won't be disappointed.  If you are choosing between a new piece of glass vs. the upgrade, get the glass instead... you'll get more for you money in terms of shooting options.

Image Quality:
The D300 offers better control and quality of JPEG images.  The white balance fine tuning includes green/magenta as well as the standard blue/yellow.  In high contrast situations, the active D lighting helps as well.  I like the color reproduction/saturation on the D300 above that of the D200 as well.  Color fringing is reduced.  Of course, if you are shooting in RAW, none of these items matter.  On the RAW note, we've got a true lossless compressed NEF format option now. Hallelujah!!!!

Active D Lighting:
Added 11/29/07 - Speaking of image quality, the new Active-D lighting deserves a bit more attention.  Basically, Active-D attempts to correct extended dynamic range problems before the image is recorded.  The camera reduces exposure to save the highlights and then takes advantage of digital imaging's ability to recover detail in shadow areas - similar to Photoshop's shadow/highlight control.  While not the most inventive composition, you can see the effect in the images below... pay attention to the roofing in the lower right-hand corner.  Active-D brings back detail in that area. 

Without Active D	With Active D

 You can see an even more pronounced effect with strong highlights in the image (such as clouds).  However, Active D doesn't always help... sometimes it can wash out images somewhat, as shown below...  again, not the most compelling image, but you can see the effect.  The separation between the area lit in sunlight vs. the area in shadow is reduced with Active-D, but the colors are less intense. 

Without Active D	With Active D

Overall, I am going to leave Active D on most of the time (when shooting direct to JPEG).  If needed, you can boost the saturation in camera or in Photoshop later.  Note that Active D lighting does not have any impact on RAW images.  (I haven't tested it in TIFF mode, as I never have - or will - shoot straight to TIFF...never understood the point when RAW is an option.)

Diffraction at High Apertures:
Every test I have run so far indicates that the D300 has superior resistance to diffraction at higher apertures than the D200.  For the math geeks out there, the question is simple.....  why?  Given the same lens, aperture, and sensor size, what else figures into the equation?  Maybe Nikon has tweaked the sharpening algorithm for higher apertures.   I need to complete some more formal tests on this topic. 
Update:  11/29/2007 - I've been able to test this more formally, and I no longer believe the D300 to have better diffraction performance.  I am thinking that the early tests were influenced by the more accurate focusing of the D300.  I ran several tests under more formal conditions (with a tripod), and see little difference in these tests.

Auto-Focus:
The D300's autofocus system is a major improvement over the D200's.  It is more user friendly - you now choose from single point AF, dynamic AF, and auto AF.   In single-servo mode, the single AF and dynamic AF work the same.  In dynamic mode, you can choose from four different sensor patterns, of which the 51 point 3-D tracking is already my favorite.  Moving subjects are just dead on.  The auto AF works much better than the "closest subject priority" found on the D200.  I've tested the AF on both AF and AF-S lenses - I am happy with the speed and accuracy of both.
Update:  11/29/2007 - The AF system in the D300 is ridiculously superior to that in the D200.  Don't get me wrong... the D200 AF system is very good - the D300 is just that much better...

ISO Performance:
In short, you get about one stop better performance on the D300 as compared to the D200 between ISO's 800-1600, and about two stops better beyond ISO 1600. 
Update:  12/10/2007 - I've added more test shots and thoughts on this topic.  I created a separate page to reduce the load time on the main page.  Click here to view the details.   

New Monitor:
The new monitor on the back of the D300 is a nice, although not earth-shattering, improvement.  It is 20% larger than the D200, but what really improved is the resolution... 3 times more in fact.  The picture quality is VERY good, especially for checking focus and fine detail.  It is bright and crisp - easy to view in daylight.  I am happy to see that Nikon took advantage of the extra screen real-estate to revamp the picture review process/options.  The new summary screen, shown below, is very cool.  The pan/zoom controls are also more useful/responsive.  There's also an HDMI out, but you'll have to buy a cable if you want to take advantage of that option. 

The D300 Image Summary Screen 

Live View:
While novel, I doubt that this version of 'live view' will catch on much.  It is quite clumsy to use, and the focusing is very unreliable.  I've played with it quite a bit, and don't much like it.  The monitor image lags behind changes in the view (or focus) just enough to drive you nuts.  The only time I see myself using this feature is on a tripod in manual focus where the set-up makes it hard to see through the viewfinder - such as macro work.  If you do a lot of hand held work that requires a live monitor view, ask yourself... do you really need a DSLR in the first place?  In my arrogant opinion, I think not!  ;)

Other Sundries:

• The PC sync and remote terminals now have rubber caps that are permanently attached to the body (list the connections covers on the D200) instead of the screw in caps that I always misplace.  Very nice!  Additionally, in cold weather these new covers are much easier on the fingers.

• The multi-pad button is more responsive, if not a tad too sensitive when entering in text.

• I haven't tested the MB-D10 grip yet, but will do so when it arrives.  While the camera is comfortable without it, I have a lot of heavy lenses, and the camera balances better with the extended grip.  I looks like the grip will have a second multi-pad.  Excellent!  The only thing I know I will hate... you have to remove it to get to the camera battery.
  Update:  11/29/2007 - the MB-D10 grip came in.  While it is easier to store when not in use (it doesn't have the protrusion that the MB-D200 has) I still like the MB-D200 better in that you don't have to remove the grip to change the second En-EL3a battery.  The second multi-pad is somewhat tiny and less friendly than the main one, but it is still a VERY nice addition.

• Early testing indicates a battery life nearly twice as long as the D200.  No kiddin'.  Hard to say for sure, as I have been playing with the monitor and menus a lot, but I was able to bang out 450 photos (with and without flash) with battery to spare. 

Other Resources:

• The best previews to date can be found at DPreview and Digital Review CA.   

• The D300 manual can be downloaded here.

• You need a new NEF codec (if you want thumbnail previews in Windows).  Just uninstall any current Nikon NEF codec you have already have installed before installing the new one.  Take my word on this, OK?  Seriously.  I warned you....