I recently switched full-time from Win XP to Linux Mint. For me, one of the big reasons that hasn't been mentioned so far is direction of the OS.
Being largely user-driven, Unix distros seem to have a more stable long-term vision. I didn't care for the direction of Windows, and with Win 8, lost faith in them coming back around to something that would be worth trying again. I've checked in on various Unix distros over the years and finally decided Mint 13/14 were ready for prime time, so set up my main computer with Mint 15.
There have been a few growing pains, but it's mostly been a smooth transition. Like others, I've found it tougher to find one perfect text editor for all purposes. I mostly use Geany but have Kate around for the occasional thing it's flawed at.