Even though I love Linux, there are still certain things that still prevent me from switching completely:
I've tried to get Midi working in Linux countless times. I've recently installed the Midiman firmware loader to no avail. Theres no fanfare when I plug it in. inputting lsusb in a terminal shows it's there, but I cannot seem to get the thing working in any of the programs. jackd runs, but nothing much happens.
TV-Out / Multihead
I have an old Compaq AP550 that sits connected to a portable TV through its Nvidia MX400 and single phono-type video lead, without a monitor. When Windows boots it automatically displays on the TV. Unfortunately on Linux, even with the proper drivers, once X is started it does not appear. It's an absolute pain to experiment in hacking xorg.conf. Things should be much easier than this! Also, on my main desktop, i cannot get multi-head working without stretching my desktop accross two monitors - I want two seperate desktops at 1280x1024!
Apart from this, I do use Linux 90% of the time. There is another problem I have too which stops my aging Toshiba Tecra 8000 laptop from being totally Linux, but I'll save that for another time.
Both these problems have now been circumvented. Ubuntu has had good dual head support with Nvidia drivers for a while now. The old Compaq has been retired and replaced with several times, due to hardware failure. The current machine is a HP D530 with an ATI card with HDMI-out, into the rear of a flat-screen TV. It works just like a monitor. I'm also slowly starting use a dual CPU Powermac G4 for music creation, so hopefully Windows will be used even less.
Update 2 (20th January 2016)
I have been single boot Linux for a long time on my main machine, but i relented some months back and got a second PC with Windows 10 purely for gaming, mostly GTA V.