PB PM - either You like it or not, it's not only Niko who thinks $600 is not worth for an upgrade from d90 and d300. I had d90, have d300s and after using this camera for almost two months now, I'm still not sure either it was a good decision. I don't see a huge improvement in AF speed - the 3d tracking and more AF points is useless for me, when I shoot animals - where I need speed. I decided to run for d300s only because of three reasons: more fps - it's damm fast with grip and en-el4a battery, dual card slot, light measurement with manual lenses. the sealing and handling is comparable (I know that one is magnesium, and has seals all around but in real life it's comparable, not to mention the buttons), and trust me I have big hands (can lift up basketball)
As for AF speed, that totally depends how many points you use, and what subject. I never use 3D tracking, it's useless for non-human subjects. In practice it is a focus and recompose tool, nothing more, nothing less. I find I get far more in focus birds in flight with the D300 than I did with my D80 or D90, thanks in part to having more than one cross type AF point. I use the center point often, but having the other cross type sensors helps me avoid focus and recompose, which I always hated doing.
When I first got my D300 I though that it wasn't worth having over the D90, but over time, in the year since I bought it, I came to see how much of an improvement it is. As you mentioned the higher frame rate was one of the big differences, along with dust and moisture seals. That's a biggy here in the Vancouver area considering that it rains over half the year. Considering that I have been caught out in heavy rain/snow with my camera, I'm glad I had such a body.
I have the D300, not the "s", so I cannot comment on the dual card slots, but then I've never had a card fail so it has never been an issue. As as mentioned by another, the bigger 100% viewfinder is extremely nice thing to have. Not to mention all the additional shooting information being shown in the viewfinder, such as the ISO. I know you could show the ISO in the D90 finder, but at the expense of knowing how many shots were left on the card. Another biggy for me as a bird photographer, the larger buffer. Being able to take 17 RAW images without slowing down vs. 9 makes a huge difference.
I use non-CPU lenses often, so being able to program them in and not guess at exposures is a big plus. The menu system and customization of the D300 is another reason I love the body. I also like having true mirror lockup for long night expsoures, it can make a difference in image sharpness in my experience.
As you can see it isn't just one or two little buttons that make the D300 better for me than the D90, it is a bunch of different things. Sure the D90 replacement might have some of those things, but I doubt it will have all of them, which would likely keep me from buying it. I'll likely hold onto my D300 until I can afford or justify an FX body, if the D7000 is the direction Nikon is going with DX bodies, and there is no true D300(s) successor.