I'll just pass along a few random suggestions, thinking back on when I upgraded from the D40. To an extent, what's important depends on what kind of shooting you like to do.
1) Part of what you paid all that money for are the extra buttons and switches all over the camera body. They make adjustment of a lot of key settings fast and efficient - much better than having to access menus like on the consumer bodies. Learn them - they're part of what make the D300 such a joy to shoot.
2) The autofocus system is a wonder, but it's also fairly complex, and it takes some learning to understand its nuances and get the most out of it.
3) The settings in the Picture Controls menu make a huge difference to how your pictures look (color saturation, contrast, sharpness, etc.), and it's important to experiment with and learn the effects of these settings, especially if you don't enjoy spending a lot of time screwing around with post-processing. If you're shooting raw and post-processing everything in Aperture 3 (indicated in your profile), then this doesn't matter so much.
4) Active D-lighting will be new to you. It can be useful in certain shooting situations to deal with shadows.
5) People seem to gripe a lot about the D300's performance under low light, especially in comparison to some of the new bodies, but if you take some time to learn how to handle it in those situations, it can do a remarkably fine job for you.
6) Get a good-quality camera strap. Paired with a pro-level zoom lens, this rig can weigh in at 4-5 pounds, which can wear on you if you're carrying it around all day.