Their main disadvantage is their slow speed. Defense matrix can let a unit take a lot of punishment and irradiate works great against large groups of enemy units. Lets you access difficult to reach areas and drop your forces behind enemy lines. The Terrans are well equipped at stopping the zergling rush. They do however have problems with early Zealot attacks by the Protoss.
Bunkers are a great thing to have.
If there is a ramp leading to your colony build bunkers there and you should be fine, for awhile at least. If your facing Protoss or Zerg you will want to get Firebats fast. These units are a great addition to your marine corps. As a Terran you have two main options - you can build up a large force of marines, the bigger the better, and assault an enemy or you can build up to heavy units like tanks. If you are a good player you can usually attack with a large force of marines and still get those tanks pretty fast.