This is certainly interesting. iMovie '08 is clearly an all new and different application, not an upgrade from iMovie '06. Based on playing with it for a short while, it is much better in managing your clips and assembling clips into a movie, but weaker on the editing side.
much, much better in keeping track of your clips, and assembling them into a movie. This part is really nice.
Clips now show as a series of slides instead of one large box with a single tiny slide in the middle. Nice.
You can scrub (i.e. moving around a clip with the mouse) blazingly fast, and you hear audio while you do that. Very nice.
new video effects for cropping and rotating. Very nice.
all the basic video adjustments (adjust color or brightness) are still there, but all the real video effects like Earthquake, Fog, Flash etc. are gone
no themes
no concept of audio and video tracks. No more extracting of audio, or dragging the audio clips to make up for an audio/video sync problem. Volume adjustments can only be done on the whole clip.
much easier to apply titles and subtitles.
tighter integration for sharing with iTunes, Apple TV, YouTube etc. You can even remove previously shared movies from within iMovie.
for new projects, you only choose the aspect ratio (3:2, 4:3, 16:9), not the resolution or encoding (i.e. no more DV/HDV/MPEG-4 choices).
You can import MPEG-2 clips directly from a USB connected DVD camcorder. Just plug it in, and a window with your clips pops up. It appears that these clips get copied directly into your project, without re-encoding to DV.
You still cannot import MPEG-2 clips from your hard disk or a DVD in the local drive.
Those are just the basics. You could probably write a whole book on all the other things that are different.