The way I play a Dom is really dependent on the circumstances of the team and who we're facing. I see them as a Swiss army knife of a an AT.
Squishier ones or low level ones tend to stand in back and mez everyone repeatedly to help keep the damage dealers safe; toss in a ranged attack or two.
More protected ones and higher level ones (as that's usually the same thing) tend to open with a crowd-wide mez and then rush into melee, esp. if they have any PBAoE like Atom Smasher.
And then, while in melee, what else needs tending to?: more mezzing, another melee blow, or a ranged attack for that runner? Melee toons just hit, maybe pop a heal or something, then hit again. I think I abandon melee toons before 50 due to disinterest the most, unless I've given them a bit of extra via power pools or whatever.