Hi doc1234,
Besides the obligatory additional goodies like added fluff, artwork, and "making-off" pictures, the rulebook does go into some more depth concerning rules, compared to the quick start rules from the Megacon boxes. Chiefly, the rulebook defines the various ACMs ("Advanced Combat Manoeuvres") in greater detail (along with examples) while also highlighting ACMs that are not included on the quick start card, including bounding, suppression, and overwatch.
However, I have provided some links below that are no longer directly available from the MERCS download page, that deal with these advanced ACMs.
http://www.mercsminis.com/sites/default/files/MERCS%20Bounding.pdf
http://www.mercsminis.com/sites/default/files/MERCS%20Supression%20and%20Overwatch.pdf
I hope that helped. Furthermore, welcome to the cause!