Tacos are a nice little snack. When I lived in San Francisco, if I passed by a taqueria I liked, I'd often stop in for a quickie - even if I was on my way to dinner; the small size means you don't have to worry about ruining your appetite. The trouble came when I'd finished eating it - I'd have a hard time resisting ordering another.
A taco is - or should be - quite austere. Just tortilla (soft corn tortilla, please, not fried!) with a small amount of filling and a little topping. Too often, tacos have too much gloop and the topping is overwhelming. The tortilla can be filled with almost anything: meat, fish, cheese, vegetables or even just sauce. It should also be quite small, so you can pick it up with your fingers and eat it in just a few bites.
Traditional tacos - such as those filled with carnitas, carne asada or fish - are excellent. Purists might disagree, but just as tasty are the fusion types, such as those made famous by the Kogi taco trucks in Los Angeles, which are filled with kalbi (Korean marinated beef), spicy pork or tofu.