system does not power off on "poweroff", just halts

Try adding

acpi=noirq

to the kernel boot parameters. This lets it poweroff on shutdown/restart (tested with kernels 4.4 and 4.7rc5).

It seems to suspend too, but unfortunately does not resume from suspend on pressing the power button.

This has worked fine for over three months now on the A740, so I'm calling this solved.

My best guess based on the info provided is a buggy UEFI BIOS. digging through the kernel bugs for Haswell I found a possible workaround. Try using xhci_hcd.quirks=262144 as a boot option or Disabling xhci in the UEFI.

The only other options I can think of are as follows:

A) Wait and hope that either the kernel development team or Lenovo comes up with an update that resolves the problem.

B) Contact Lenovo Support and push for a BIOS update that resolves the problem or encourage others with the same problem to subscribe to your bug report. This may or may not be any more effective than A.

C) Modify the BIOS or the kernel yourself until you reach the desired result (Not for the faint of heart). I'm not recommending this course of action, only including it for completeness. Modifying the BIOS can easily leave you with an unbootable system with a voided warranty. You should also read carefully the reasons for and against compiling your own kernel in the aforementioned linked document.

Source: https://bugzilla.kernel.org/show_bug.cgi?id=66171#c118

After ferreting through the system files I saw a few warning about the BIOS. I checked Intel's website and there was an upgrade available that seemed to solve an issue of overlapping memory addresses. Not obviously the same but my logs indicated that various sectors of my BIOS were returning unexpected values, which did not prevent the kernel from starting but obviously was not good. The issue was not apparent until the kernel stopped using upstart and started using systemd.

I downloaded the updated BIOS and applied it and now my system turns off as expected.