Typically, the following units can gain "critical mass":
- Marines (with Marauders)
- (Blink) Stalkers
- Siege Tanks
- Void Rays
I've played several 2v2 and 3v3 games where one person masses one of these units and pretty much walks over bases like they weren't there. To counter this strategy, you must SCOUT OFTEN.
Massing is a poor strategy if you can build counters against those units in time.
The other game aspects I can mention are key to success in any game:
- Pressure your opponent with harassment or even trade units to keep numbers down
- Expand more often than your opponent
- Defend against his harassment better than he defends against yours.
- Force him to build other units than the ones he'd like to mass, to defend against your harassment