I upgraded to Win10 yesterday. I also got the new nVidia Win10 driver for my card (GTX 770) and did a complete reinstall using the custom / clean install process. The only thing I left unchecked was the nVidia system manager or whatever that thing is called and only installed the drivers and the PhysX, and not the manager.
I also found I had to reinstall DirectX9 because both Rift and GW2 seems to require this older DirectX9 to run. I had already set my Win updater to NOT install any graphics drivers for me. That is the problem....whenever Microsoft sends out these updated drivers, they are not necessarily the ones my card uses best and can really cause issues.
But so far so good...everything looks and works great...except for the game lag that seems to permeate this game no matter what you do. Game engine God hates to let things run smoothly.
One of these days I hope Trion just builds a better game engine that comes into the 21st century and keeps up with the developing tech properly.