I'm not sure if my ears are just bad, but I notice very few differences at all, not even Texas or California. I've lived in Chicago, Atlanta, Houston, and Los Angeles, and I've very rarely run into people who sounded like they had an identifiable regional accent in any of them. If you go to rural areas it changes—West Texas has some Texas accents—but suburban Houston sounded very close to suburban Chicago to me, except for a few lexical differences like "pop" vs. "coke".
There's a bit of selection bias due to the fact that you are reading HN. I can promise you that the small percentage of people who work in the software field are a type of educational elite, and you are, generally, going to live in places and work in offices with other members of the globo-corporate elite. Also, just to be clear, MANY, MANY people who speak regional dialects at home or informally will shift into a northeastern, "business standard" dialect when in a work setting. I should know. I am one of them. My Appalachian accent does me few favors in business environments.
I'm young enough that I'm thinking more of people I went to school with in most of those cities, not really professional colleagues: elementary/middle-school in Chicago, and high-school in Houston. But it's true that they were middle-class suburban areas, and probably also had a lot of people who moved around (few of my Houston classmates were multi-generational Texans).
A little bit, but not very much. That one did differ in any case: most people in Chicago I knew would say "you guys" for the 2nd-person plural, while most people in Houston would say "you", with a minority usage of "y'all".
It's mostly accents I didn't notice, though. Some wording changes, yes, but nobody in Chicago I knew had a "Chicago accent", like the Superfans on SNL, and nobody I knew in Houston had a "Texas accent". I could be missing more subtle differences.
