Thought I'd report back after using Win8.1 for a day after so much time on Win10.
The good: Wow, so THAT'S what the SP3 is suppose to act like. From picture login working without issue, to sleep/hibernate actually _working_, to real snappiness, to the fan only running when the machine is actually doing something important, Win8.1 is such a stable OS on the SP3. I now remember why I liked the SP3 so much when I first purchased it (and when some of the initial kinks were worked out): it's a very nice-running system. The most recent official Intel HD drivers for Win8.1 even have the deep black levels that make such a huge difference in darker video (read, Daredevil on Netflix). More than anything, going back to Win8.1 for a little while shows me just how much work remains on Win10's performance and stability.
The not-so-good: I'm actually pretty ambivalent about Win8.1's UI overall. Yes, it's definitely more consciously designed for touch, but I just don't like the full-screen Start. I prefer Win10's Start menu with its vertical scrolling (seems more natural to me), even when just using touch. I also don't really like the swipe-from-left multitasking gesture as much on Win8.1, or the lack of a close button on apps. The Charms are okay, I guess, but just really _different_ than anything else that's out there, and just a little jarring to use. Overall, Win8.1 is just _weird_, more than I remembered. Microsoft really did try to change paradigms with it, and obviously they didn't succeed.
The bad: I don't like how Desktop apps are all held in one multitask window, so to get to a specific desktop app it's swipe-from-left, then find and open on the desktop. Modern apps are so much more useful when they coexist more naturally with Win32 apps, i.e., windowed and with the same controls. Constantly switching between full-screen modern apps and to/from the desktop just isn't a smooth experience. It's not exactly a knock against Win8.1 directly, but the newest stuff is only on Win10 (Office Preview, Music Preview, etc.), and I do miss it a little.
I think Win10 is going to ultimately be preferable to me once it actually performs acceptably. I suppose I'm just a little concerned that it performs SO poorly on the SP3 at this point. So much work to be done.