The best way to describe Tomatillo is "Cali-Mex." It's the bright fresh interpretation of Mexican popularized by American chains like Chipotle. The tortillas are made in the store. Sometimes you can see the dude making them. The ordering style goes where you choose your meat and then its vehicle--taco, burrito, quesadilla--and the type of tortilla. The flavors aren't heavy like On the Border and other Tex-Mex places. They also serve alcohol and frozen beverages. I felt the margarita was a bit heavy on the sour mix and light on the alcohol. They also serve a great fish taco.
UPDATE: It's been years since this review. The Mexican Renaissance has since happened in Seoul, and Tomatillo has been pushed into the ranks of fast food Mexican. It's opened a few more stores. The biggest complaint is the over-reliance on flour tortillas, even to make chips. They are decent for an occasional lunch. The salsa always has some funky flavor, like it's been sitting in a dirty container.