This.
Even as far as three months ago, Windows 10 Preview was demonstrating a significantly more fluid OS than Yosemite. Windows 10 has shown even more improvement in the fluidity of the UI over the past couple of weeks. It's a good think Apple is focusing on squashing bugs and improving performance with their next OS release, the fact is they need to in order to keep up with the competition.
What concerns me is how Windows 10 will behave like a tablet when the keyboard is detached, and like Windows 7 when the keyboard is attached. In a lot of ways a Windows 10/Surface Pro 3 is going to be the best of both worlds.
Even with a more fluid Windows 10 UI vs. Yosemite, I still prefer Yosemite by a small amount, but I feel that Microsoft is starting with an OS that as a much stronger foundation. With Windows 10 so fluid, with such a small footprint, and more efficient in terms of memory usage, they'll be in a position to refine the UI as they please since they are now following a different model in terms of updates. Basically, Microsoft has finally focused on user experience and taken feedback from customers, and it shows.
Tim isn't going to have the luxury of pulling his team off of OS X development to work on the next Apple Watch project and then spare OS X development quality. It's not like Microsoft has to focus on multiple devices, and now they don't even have to focus on multiple OSes, their tablet OS is the same as their desktop OS which is the same as their phone app. I really think Microsoft is the one skating towards where the puck is this time, not Apple. Let's hope Apple is developing something and just keeping silent about it (which is possible).