1. It is almost hopeless to actually edit HD in iMovie HD unless you have a dual G5 with 27 GB of RAM, even though all the Apple literature, their sales people, and the guy at the camera shop said that a Mini should be able to handle HD, no problem. I have no problem editing regular DV, but HD just brings the poor little thing to its metaphorical knees. If I try to import more than about half an hour of HD at a time (which takes four hours by the way because a Mini can only import HD at 1/8 speed) it hangs the OS X kernel. Even the mouse stops moving.
2. There is no way to add a simple block of text in the center of the screen in a single font size that doesn't move around in some obnoxious way. For a company that prides itself on minimalist aesthetics this is an unforgivable omission.
3. If you have split a clip into two pieces there is apparently no way to join them back together again (if, for example, you cut something too short for a transition that you want to apply).
4. Rendering effects takes forever, and there is no apparent progress indication. (I'm still waiting for a fade to render as I write this.)
5. There are a bunch of features that have visible controls on OS X 10.3 but which don't actually work unless you have 10.4.
Bottom line: if you're considering stepping up to HD, don't do it unless you're prepared to fork out for a brand new top-of-the-line Mac with OS X 10.4 and Final Cut. Anything less will just cause you pain.