If you want the real Mission District taqueria experience, you can't get any more real than the Taqueria Altena (should be a ~ over the n). It's not primarily a gringo place, unlike some taquerias that you hear a lot about. It's not self-consciously cute. It is very basic, but it has wooden tables and chairs and the food is very consistent and authentic. Have one of their aguas frescas or carrot juice to drink. Unless you're a snob, you'll feel comfortable there. They actually have 2 places, one in the Mission Mall and the other around the block on 22nd St.. But the one attached to the Mission Mall on Mission St. is the original, the busiest and most colorful. It's fine for small groups but it's not the kind of place where you would celebrate a birthday.
Pros: fast tacos, burritos, public transportation, authentic
Cons: limited menu