the biggest deficits in the D90 are seen when it's used in a truly professional setting (which 90% of people aren't doing). These deficiencies are:
1. 450-pt Matrix Metering vs. D300 1005-pt.
2. AF-speed (due to matrix metering differences in #1)
3. AF-point coverage
This is significant for the following:
1. Sport shooters - have you shot a figure skater and had to track, recompose, and shoot at high FPS with a D90 vs. D300? I struggle with the far superior D3's AF, so you'll notice differences for sure.
2. Event shooters - have you shot a dance sequence at a wedding, or attempted to capture those candid moments for your clients? With the superior AF coverage and AF-speed of the D300, more doable than with the D90.
3. Wild life shooters - same as above, intimate settings with animals are difficult to stumble upon, superior MM will allow more accurate metering, better shots, and of course better results for capturing those rare moments.
Just my thoughts - for that "extra" $500, it's worth it to professionals since it means getting the shot or not.
I'm actually looking forward to the D400 iteration as a backup to my D3, hoping the noise results are the same, the extra reach will be more than welcome, and a large AF coverage is exciting for me.