For the longer flights, you can be sure there would be somewhat of a weight restriction, even on the 738's and 753's. When I worked at America West, we would occasionally have a slight weight restriction on the A319 to
LAS when the weather was really warm. But this wasn't a regular occurence. I'm sure that ATA might have weight restrictions on their west coast runs but it probably isn't a regular occurence either, since the 738 and 753 have a real thrust/weight ratio so they can easily take-off and land out of
MDW no problem. Back when ATA ran 727-200's out of
MDW, there were a few times when the flights would have to make a fuel stop (Lincoln, NE for example) in order to make it legal to takeoff.
As for which runways are used for commercial a/c ops, it's only 31C/13C and 4R/22L. The other runways are strictly reserved for
GA.
MDW is not in a bad neighborhood at all. I lived about a mile east of the airport in the Westlawn neighborhood, and it was probably one of the most underated places to live in the city. Wide streets, nice affordable houses, plenty of parking. One of the best places I've lived in while in Chicago (and I've moved around the city and burbs quite a bit

). The area a few miles to the north of the airport is a bit shady but not exactly the ghetto. And you can leave your car in long-term without having to worry. I've done it in the past with no problem.