My friend Lauri made this request of me last night, and I have to say, its really quite a challenge!
My first impulse would be to say Sushi, but Lauri actually lives in Japan, so the sushi here doesn't really compare. (Although there is a lot of great sushi in New Orleans: Horinoya, Shogun, Ninja, and Little Tokyo (in Metairie) spring immediately to mind. Of course, one has to avoid sugary stuff like eel sauce and snowcrab, fried stuff like crunchy and tempura, and of course the spicy mayo- but overall I think that there are a lot of great, light options to be had on the Japanese menu.)
There are a couple of great light chains that have popped up in the New Orleans area: I personally love Roly Poly (and love that they give you full nutritional info for every roll on their website) and I've heard good things about the Pita Pit.
I think you can also get by pretty lightly at Vietnamese restaurants if you stick with light fare like spring rolls and rice noodle salad bowls. I like Pho Tau Be on the Westbank and Pho Orchid in Metairie.
I think you can also do well with Middle Eastern food. I really love Lebanon's Cafe and Mona's Cafe.
Finally I think that Tapas can work out well for eating lightly. First of all, the portions are small, so you can order fewer plates if you aren't looking to eat very much. And in my experience there are often a lot of vegitarian and seafood options on the menu. There is certainly rich fare to be had as well, but that's always the case when you're eating out. I love all of the tapas restaurants I've been to in New Orleans. Try Vega Tapas, Baru, Cafe Granada, and Mimi's in the Marigny.