What makes Mini-Dv superior to the Digital Hi-8 Format?
Basically, not much.
They both use nearly identical codecs to record the information to tape, and the picture quality/resolution is very similar (if not the same) on similarly priced MiniDV cameras and Digital8s.
The D8 does have a couple of advantages - the main ones being that as it uses regular Hi-8 tapes, they're cheaper to obtain than MiniDV cassettes. Also, it means that if you've got any old Hi-8 films that you want to transfer to a different format, or edit, then you can do that very easily.
However, there are disadvantages - the first one being that as D8 camcorders run Hi-8 tapes faster than they would be run in an ordinary Hi-8 camera, you only get about 40 minutes of filming on a 60 min casette, which can prove to be a problem. The other main disadvantage is that as MiniDV has been adopted as the 'standard' format, it's possible that Hi-8/D8 tapes will cease to be produced in the near future, which could render your camera pretty useless.
I hope this helps