I would suggest Osha Thai near 2nd & Mission. Maybe a 15-minute walk from Moscone. Not totally authentic but tasty and popular.

For good Vietnamese (or more pan-Asian) without breaking the bank, try Out the Door, which is in the food court in the basement level of the Westfield Shopping Centre. Just a couple of blocks from Moscone.

Kokkari in the Financial District is well-known for Greek, though it is slightly on the pricier side.

If you are willing to venture up to Chinatown (longer walk or short bus ride), there are many authentic restaurants to choose from. One tourist favorite that I like (not authentic but unique and tasty) is House of Nanking.

Near Chinatown is North Beach, with lots of good Italian restaurants. I would recommend Tommaso for pizza.

I'm not aware of any good Mexican places in that area. There is a Chevy's nearby, but that's just a Tex-Mex chain. I would suggest taking the BART to either of the Mission St. stations (16th or 24th) for a mighty fine, S.F.-style taqueria experience. One of my favorites is La Taqueria, which is just a block from the 24th & Mission BART station. For an upscale Mexican meal, I would suggest making reservations at Colibri in the Union Square area.

For Indian food, Amber of India recently opened up a location very close to Moscone, near 4th & Market. I have not eaten yet at that location, but I have eaten at two of their other locations, and the food was very good. Alternatively, if you are willing to venture to a seedier part of town, try Shalimar in the Tenderloin area. It serves Pakistani food (very similar to Indian). It is a dump, but the food is very good. And super cheap. I would suggest taking a cab, unless you are personally comfortable with taking buses through inner cities.

If you like Persian food, I would highly recommend Lavash in the Inner Sunset. It is nowhere close to Moscone, but you could take the "N" Muni line directly there (about 25-30 minute ride, probably).

As an aside, Yelp.com is generally a good resource for SF restaurants.