Probably lots of cuts on your calves an shins to start with.
Feet bouncing of pedals through braking bump style territory (on a HT).
Having to think about weight and bike control a bit more but getting more out of it.
Not getting mashed up by your bike if you stack it.
Not being any slower up hills.
Not losing the ability to have a smooth pedal motion.
A transition period where it feels weird. Stick at it.