Disclaimer: I'm the furthest thing from an expert.

The hallway in my house was filled with these nails, over 75% of them were not even hitting joists. Could not pound them down because the heads are so thick, coupled with the fact that they are not in a joist, they just bounce right back up. Wasn't fun, but I think it was the right thing to do. This way I can sand and plane and not hit anything; once it's about even then I'll screw it all back down, and double-check and touch up any other problem areas.

If I'm correct, screws should be used, and if not screws, at least ring-shanked nailes, like about 8 of them are in the pic?

Those are 8d CC (cement coated) framing nails. Thats what most builders were using. Better to use screws or ring shanked / screw shanked underlayment nails.

For some reason most of the house was done with the ring shank nails, but it's like as if they ran out of nails in the hallway and just used these. Had to pull them all out because I couldn't get the heads under or even with the surface, and I have a bit of planing and sanding to do. I'll just screw it down after.

How can you tell they're cement coated? They felt like shiny smooth metal to me?

After the house was built, a supervisor probably told a carpenter to go nail down any subfloor squeaks. So novice "carpenter" ends up driving numerous #8 framing nails into the subfloor attempting to remove the squeaks. OR, it was done by a carpet layer for the same reason. Screws are NOT required or customary in track housing. Typically, in newer homes, the subfloors will be glued and nailed down with pneumatic driven ring-shanked nails.

I bet you're exactly right. Makes sense that they use pneumatic nails now that you mention it. Screws are too time consuming. I've already got a drill so I guess I'll just stick with screws for now.

You can bank on it. After nearly 30 years on the floor, I'd say I pretty much understand what's happening.

On a house that's ~25-30 years old, about how much time do you figure is spent on preparing the subfloor? Is it common to have humps that have to be cut all the way through a ply of plywood? Just sanding this floor down has taken me a couple/few days (few hours at a time)...It must suck having to go through this at every single darn house.

It happens; not on every home but enough. In track housing, production is the name of the game. Many phases of the construction of each home are "subbed out" to piece workers. They get paid on how much they get down and not by the hour. So many corners are cut. Since the homes have to be inspected by the local government to ensure minimum code compliance, the "piecers" make sure that their work will pass code. But codes are not written for quality but safety. So while the joists will be properly nailed and the spans won't exceed codes, those joists maybe (and many times are) uneven in width and aren't crowned when installed. Consequently, one gets an uneven subfloor. Add to that poor foundation designs and/or improper grading and expansive soils and you can have a foundation that's moving around, also creating uneven subfloors.

If I get on a job that's really bad, I may just get another general contractor to deal with it. Sometimes, it has been a royal PITA.

Be glad your not on concrete, with the humps and valleys!!!!