You don't have to use the cortana thing at all (and I really wouldn't).

Windows 10 is full of bugs, but that's kind of the point of the technical preview, they're pushing out ideas and features to the public so they can basically troubleshoot everything. There are some nice things, and some not so nice things. Not but just a few days ago, they updated it after a little bit to fix the text in a lot of windows programs, which until now would mess up graphically horribly. And with that fix now my e-mail and calendar stuff doesn't work. It's not meant if you want a perfect operating system right now, and they've promised it's probably going to break even more for now.

It works fine to play games, at least mostly fine. As far as FFXIV it's just as good as W8 or W7. I have two cards running in SLI and it's perfectly compatible with that. However I've tried running a game that uses Gameguard, and Gameguard doesn't recognize W10 as a valid operating system and so it is not compatible (I had to install W7 on a separate hard drive to get around this).

As far as the future goes, when it's released (assuming they have all the bugs fixed), I foresee it being a pretty nice OS. There's a real fear they'll focus more on the mobile features and stuff, but so far it isn't gone too far into that.