Mexican food in Decatur falls into three categories: taquerias, specializing in quick, cheap plates of tacos and quesadillas; burrito joints, which vary from the most basic beans-and-meat approach to Raging Burrito's thousand-option menu; and upscale Mexican places with pretensions to cuisine. Zocalo is in the last category: it offers a range of non-burrito, non-taco Mexican options, plus the usual elaborate Margarita bar.
We have two favorites when we go to Zocalo. Paka gets the shrimp fajitas, which give you a large pile of shrimp fried with onions and sweet red peppers, together with tortillas and a goodly plate of condiments. I get the enchiladas verdes, one of the two vegetarian options on the menu: three slender cheese enchiladas (almost flautas) in a thick, zingy green tomatillo salsa, served with beans and veggie-studed rice.
Zocalo's main attraction from my point of view is the guacamole, which your server makes for you at your table. For about $4.50, your server will bring a shallow black stone bowl, which stands on three stubby legs, to your table, and pound a whole avocado in it with the ingredients of your choice -- onions, tomatoes, jalapeño peppers, etc. This is very satisfying to watch, especially after dark, when the restaurant is primarily lit by the small parafin lamps on each table, and the guacamole-making takes on a faint echo of Aztec ritual sacrifice.
Or maybe that's just the margaritas talking. Zocalo makes a good house margarita: tangy, with more flavor of fruit than usual, available salted or unsalted, frozen or on the rocks. They've also got a wide array of beers and some truly terrible sangria. (Nowhere in Georgia seems to make adequate sangria; it always tastes like bitter Kool-aid. If you know of any good places to get sangria within an hour's drive of Decatur, please send me a message.)
Like many places on the Square, Zocalo tends to have a youngish, blondish, richish-looking crowd on weekend nights. It's not exactly a hipster hangout, but it wouldn't mind being one. The outside tables can be hard to get, but inside is nice too, if you don't mind the dark (interrupted, more than illuminated, by light bulbs hidden behind a display pierced-metal screens near the door).
Zocalo's prime Decatur competitor, as far as I'm concerned, is Billy Goat's Tavern in Decatur. Both are good places: which one you choose depends on what exactly you're after. For shrimp, fajitas, hipsters, and noise, choose Zocalo; for tamales, mole sauce, hippies, and cushioned-bench atmosphere, choose Billy Goat's. For me, that swings the balance just barely in Billy Goat's favor; for Paka, who loves shrimp, it goes the other way. Your call.