I would say the two biggest things for roamer would be: 1) understanding when it's the right time to go be a big idiot all alone or when to fall back and just stay alive and 2) having confidence in your scouts/your scouts having confidence in you.
Random example: Say you have 80 hp and you're a rocket jump away from a health pack. At the same time, you notice your friendly scout is fighting another scout on your teams turf. Even though you're low enough to get one shot and there is a tantalizing health pack only seconds away, you should probably rj over and help a brother out. Even though you really don't have enough health to do much, that enemy scout doesn't know so he'll probably continue trying to fight your friendly scout and die to your rockets. In the extra time you take to grab health your friendly scout might win the fight, but he could lose it just as easily if you're not there. tl;dr Pussyitis (chrome isn't telling me to spell check that I'm rollin with it) is the leading cause of Roamer Deficiency.