HD Video Mode
When the D90 was launched, the headline feature on the spec page was the High Definition movie ability. Although this has since been replicated in other cameras, the D90 was the first camera to have it. In practice it works well, but is not without its drawbacks. Press the Live View button, and then the 'OK' button to start recording. The D90 records at 720p and 24 fps which is good, but not quite as smooth as the 30 or even 60fp fps you will find in many point and shoot cameras these days. The much publicized flaw you may encounter is known as the jello effect. You experience a wobble or 'jello' effect in your videos when you quickly pan horizontally while shooting a movie. Also worth noting is that the D-movie mode is manual focus only on the D90, regardless of what lens you have on. However, all that aside, you can get great results with some practice. Check out the D90 channel on Vimeo for some great examples of what can be done.
The 3-inch LCD is great for reviewing images, and even doing some in-camera editing. There are a whole variety of effects you can add to your photos, like red eye correction, fisheye effects, distortion correction, black and white conversions, various filter effects, picture straightening, and an in-camera RAW to JPEG conversion facility. Although I do many of these things in Photoshop, it is often good to see what it would look like before you get back to your computer. The D90 saves the retouched photos as separate files, so you will always have your original to go back to.
Nikon's Active D-Lighting is a feature originally borrowed from the D3, and D300. It helps ensure your highlights are not too bright, and your shadows still have detail. You can apply D-Lighting in a variety of strengths, turn it off altogether, or just leave it on auto to let the camera pick the best settings. I have mine on auto, and almost always get the results I want.