The D90 is able to do the job, most of the time, but honestly it just wasn't cutting it for me. Everyone's shooting style is a little different, and I like having the controls at my finger tips. I found more buttons very useful, no more digging into menus to complete simple tasks. I use the different shooting banks a lot, so having that feature is important to me. Plus I found the focus tracking of the D90 unable to keep up with many birds in flight. The ergonomics of the D300 fit me better to, my fingers were always getting pinched between my lenses and the camera body with the D90.
I also like having the weather resistant body, with all the rain we get here in the Vancouver area, not having one keeps you indoors a good chunk of the year. Yes I could use a plastic bag over the body, but I found that to get in the way too much. So I'll stick with the D300 thanks. ;-)