Hello,
Here are (some) of the main differences :
MJ8 has more organization features. You can now select from a lot of fields
to design your Library Views, including Custom fields.
MJ8 also allows you to organize your Audio CD & DVDs collection.
MJ8 has more flexibility in searching tracks and managing playlists. It adds a
lot of Keywords and rules for the search/smarltist engine.
Example : "~t60 genre=Jazz" creates a 60 minutes randomly picked jazz tracks
MJ8 has a powerful visualization scheme including a nice visualization studio,
a variety of track info templates and more...
MJ8 has a new plugin interface and allows for Developpers to design great new plugins.
This only is a real change from MJ7. To have a feeling of what it can offer
check Rhinobanga's Album View plugin, fantastic!
MJ8 has a new skinning engine for both Mini and Mega skins.
Check lowerleft by Zrocker, a must!
I probably forgot some features. Maybe folks on the interact and at JRiver
will add a few.