I am a very heavy daily user of Win 10 TP, for both professional and recreational purposes. Note: I am also a daily Linux user since 1996, and have no shortage of experience with OS X. I find the Win 10 UI more than acceptable, just to get that out of the way. Here are my serious requests, both of which have been submitted.
1. Fix local searching for files. The instant search works for (most) applications and (some) registered document types, but searching for unregistered files by filename is utterly broken. I use GNU find under Cygwin when I really need to, and I should not have to.
2. Give me (back) more manual control of Windows Update and Windows Defender. I should not have to go into the Scheduled Tasks administrative tool to control when these processes execute. Further, let me exclude things from Windows Update. The x64 8.1 driver for my (AMD) video card is more stable than the update provided by Windows Update, and yet weekly Windows installs the driver that I do not want.
Those are really my main requests. Overall, I find Windows 10, even at preview level, faster, just as stable and just as usable as Windows 7.