Roc uses perceus, which is a reference counting model that allows for opportunistic in place mutation if it is safe to do so. HVM is more like a fundamentally new evaluation model that is parallel by default. They are both very exciting, but HVM is much more radical and probably needs at least a few more years until it starts to be seriously practical.
Many other things are features or bugs depending of your preferences. For me, for example, eager evaluation is a big improvement, but YMMV.