To echo others, ABC commisary is easily the only "bad" meal I ever had at WDW, save for that time when I was 10 years old and got a "burger" at Villiage Haus in MK - back when it was a "turkey" burger and I didn't know until I bit into it, hehe. I too had chalked it up to a bad day there (ABC), but I haven't met many people that have liked it. It's all the same fast food, really - you know the burger you eat there came from the same place every other fast-food burger in the parks come from - but it's just not cooked well here, and every "basket" I've seen had both the burgers and fries sort of smashed together (what few fries they put in).
I'll have to try Backlot next visit, but for the "light" meal you are looking for I'd try Starring Rolls, which is right next to Brown Derby. You can get an individual sized Cobb salad there (I believe they actually make them at the Derby next door), or other "lighter" fare. Or, I'd do as a few others suggested, and get a PS at Sci-Fi or 50's PTC and split a couple of appetizers or an entree. If you pick Sci-Fi, I can only really reccomend the burgers - but the portions are large enough that splitting one and getting shakes to drink would be practical.
Counter service at MGM is easily the least attractive in any of the parks - the only thing I buy from that mess of vendors across from the Fantasmic! theater is a bottle of water (it's nice there - usually so cold there is ice inside!). I usually do one sit-down and one counter service during each full park day, and at MGM I usually just settle for a nice sit-down at my favorite place on property (Brown Derby) sometime mid-afternoon and then get dinner somewhere else after the park closes.
N.E.D.