He's solid in all facets, from skating through vision, to passing, positional play, hitting, leadership, and poise. He's thought of mainly as a mobile guy, but his defensive play is probably a touch underrated. He's a two-way guy, who has been a captain of his team in the past. Green had a very, very strong rookie year in the AHL, and played regularly en route to the Bears' league title.
I don't know if he's #1 material, but it doesn't seem a stretch at this point to think he could be a #2 someday. He should be a lock to develop into at least a top-four defenseman, and may be there by season's end on a Capitals blueline that is not terribly deep. He's likely to become a PP quarterback, given time, and has a high panic threshold.
Considering where he was picked, a very nice pick. The only thing he doesn't have is towering size, at roughly 6' 1" and 200 pounds, and that's probably why he slipped at the draft, along with playing for a struggling Blades team.