probably the main difference in quality is down to the fact that a high budget movie like MI3 has better cinemtography than your holiday movie. Don't know about you but I don't take a 60 person movie crew with me on my holidays
Lighting angles sound, lens quality etc etc all have a bearing on quality.
However, if you're saying that the original capture AVI from your camcorder is much better than that of your final export then it is quite simply down to encoding settings. What else is there?
If your workflow has you rendering and exsporting and than taking that export as a new source for rendering and exporting again then you can get degradation then but I suspect it's all down to export settings. No silver bullet here I'm afraid. You've just got to play and find the right combination of settings project by project.