Crosses the Line Twice: It certainly helps that the first name they went under was Burn The Priest.
Epic Riff - "Laid to Rest", "Redneck", "Omerta", "In Your Words"... quite a few, as you can tell.
"Grand Theft Auto" Effect -- More specifically, the Guitar Hero Effect. Many people only know Lamb of God through the appearance of "Laid to Rest" in both Guitar Hero 2 and Guitar Hero: Smash Hits (the original version, as with "Killing in the Name"). You can't really blame the people playing the game for wanting to hear the real thing (which would also appear as downloadable content for the Rock Band games).
There's also the case of others who were introduced to the band with "Black Label"'s inclusion in Tony Hawk's Underground 2, though in a minor scale than "Laid to Rest".