Since you used to shred the gnar...what is it that you like about mountain biking?
-Do you like spinning out lap times like a geek?
-Are you into exploring the trail and gawking around at the wooden thingys with leaves on them?
-How bout popping off everything that has a lip...tweaking it out in the air?
There are different kinds of bikes for different styles of riding. I break crap, so I have a gnar frame with an All Mountain build (somewhat beefy) for general trail riding, XC, shredding the XC gnar. I tried XC bikes...but I broke 'em.
I hunch that your BMX/Moto roots could cause you to be more of a "fun" rydah...versus the Madd Spinner wearing the HR monitor.