Well there are two things, one of which is philosophical, the other practical. 1. The window's experienced is designed to be just easy enough for anyone to use, but just hard enough for a thriving IT and tech support industry to make money from it. The OS is designed for you to be compelled to buy things. That is something I sincerely do not like. There are also a lot of very poor design decisions in all of the OSes, simply because they can't test enough. A lot of their underlying technology (NTFS, cmd, dlls, installation) are very badly designed, and can't be changed because they are a standard. 2. I am huge on open source software. I will always look for something open source before using closed source. That way I know that if I get into trouble, I can go to a better community, and I can fix it myself. Windows has a long history of being, well, not so nice to our open source community. I've grown to dislike their OSes, and tend to use anything but.
Oh I guess I didn't address that directly. To be completely honest, the UI and compatibility are my main issues. I program embedded systems and a lot of the software I use has problems with W8 for one reason or another. Just the fact they had to change the name of metro because it had such a negative connotation is telling of the UI.