What stops you from sinning in heaven?

[Neo-Scholastic]

So we will be limited to......loving god? I mean, sin 'as we know it' covers many aspects about being human. If we take those away, what will we be in heaven? Mindless winged robots serving the grand-master?

Not at all. You will be yourself only more like you really are on the inside. Freed from the obsessions and compulsions of sin you will be more capable of helping and serving others. Loving God also means loving your neighbor as yourself. You will have infinitely many ways to serve the Lord: guiding and protecting the living and ministering to the damned in hell to minimize the harm they do to each other.