And, as an added bonus, you can have major shoulder problems for life from lugging that 17" behemoth around.

Hah, I agree...But, on the other hand, you can carry it when needed, and I do regret not buying one of those for work instead of Razer Blade, "because it's thin and light" - it gets overly hot and noisy when doing heavier work and is definitely not up to par to these...

What operating system does it come with?

I think Lenovo (together with other questionable actions that alienate their userbase) removed the option to choose Linux for workstations, so it comes only with Windows 10. But Thinkpads are traditionally very well supported on various Linux distributions.