After thinking about this a bit, I'm prone to treat the swarm keyword as a mega-trait rather than a new role. "Half damage from melee and ranged attacks" is essentially increasingly high damage resistance, so I wouldn't want a stat block to have the swarm keyword and other extras like recharge powers or regen or whatnot.
The swarm text in MM3 and MV don't mention opportunity attacks, which presumably means that New swarms follow conventional OA rules. (They can still share space with other creatures, and it's still difficult terrain.)