I have started using GRASS 7, mostly / initially because it handles vector layers much better then GRASS 6.4. But is also has a bunch of new features.
What also changes are the options / syntax of some of the functions. I have already mentioned the changes in how the overwrite option works in r.mapcalc. On the GRASS GIS Tracker and Wiki, you can find a succinct list of changes in option settings and names.