We've got New Camera Week coming up, so just a word of warning: prepare for the worst. And prepare for the best. It was the best of pixels, it was the worst of pixels...
When new products come to market, the ones with lots of buzz all go through the same see-saw of reaction. Nikon has managed to hype the D300 and D3 so much that it is nearly inevitable that there will be deflation when unreachable expectations are not met. I call it the Law of the Sine Wave: it's hot, it's not, it's hot, it's not. The Nikon launches produced a lot of "it's hot" so be prepared for a lot of "it's not" posts across the Internet in the coming weeks. Eventually, the real story will start to emerge, and we'll be back to "it's hot" part of the wave, though perhaps without quite the amplitude. Eventually, some other camera company will launch something that gets the new buzz, and we'll be back to "it's not."
In particular, there seems to be a "best DSLR ever" expectation with the D3. From a physics standpoint, I doubt the D3 exceeds the Canon 1DIII: the sensors are close to even in terms of electron well capacity for any given area, and the read and dark current noise floors are likely to be similar, too. Both cameras live very close to the current state-of-the-art in sensor design, if not right at it. For JPEG images, differences you see will be essentially DIGIC versus EXPEED. For raw images, it's going to be the converter that supplies any meaningful difference.
Thus, in a week or two we'll have a few people claiming that the D3 is "everything it was promised to be" while most others claim they're disappointed that the D3 isn't really a big leap past Canon. My suggestion: just use a state-of-the-art camera and be happy, whether it be a D3 or a 1DIII.
The D300 is likely to get the opposite reaction (more acclaim than disappointment). Why? Because it hasn't been hyped as much and it really does move the bar for the small sensor cameras. Personally, I'm impressed that in two years you can go from 10mp and an acceptable ISO 800 to 12mp and an acceptable ISO 1600. Moreover, you've got a better AF system, a better metering system, and a faster frame rate all added into the mix. The D300 is likely to get more positive buzz when people actually start using it, especially those gravitating from the D200. Given that the D200/D300/30D/40D group is the bulk of the serious amateur market and a signficant chunk of the pro market, that's a bigger thing than Nikon equalling Canon's image quality with a high-end camera.
So, you heard it here first: the new cameras aren't as good as you thought. Wait, they're better than you thought. No, they're worse than that. What are you thinking, they're better...