The launch is still pretty new, so word is still getting around about all the details. I can confirm that dedicated servers do include MySQL — in that sense, they're basically a web VPS and a MySQL VPS glued together.
You can create users through the control panel, and they'll all get their own home directory.
The control panel is DreamHost-branded. We don't currently have any support for adjusting or removing that on a per-user basis — if nothing else, the company name ends up included in a lot of messages.
You can set it up either way, but nameservers under your own domain are less well tested, and can have some odd caveats. I'd recommend sticking with the default DreamHost nameservers unless you have special needs which preclude that.
No problem at all.
I'll have to check on that. We don't support disk quotas on shared servers or VPS, but dedicated might be a different matter.
MySQL is local on dedicated servers.
We're likely to offer dedicated servers with larger disks, or with disk arrays, in the future. Our main concern is that we'd like to avoid having too many different hardware configurations, as that'd make it more difficult for us to provision new hardware, or replace faulty hardware, in a timely manner.
No limitations besides what your server can handle, and any applicable laws.