Warning! This post is not intended as flame-bait! It is merely to present and discuss issues often overlooked when people begin to look at the usability of Linux for gaming. I use Fedora 21 as my daily driver, I am not a Linux hater (: Also: this thread is directed at issues present to native Linux gaming. Not virtualizing Windows.
If you know of a solid workaround for anything mentioned on this thread, link it!
My first and foremost issue with gaming on Linux, other than driver performance, is peripheral support. Most guys who just wanna try Linux out to see if it is practical for them don't wanna buy a new mouse and keyboard just to make sure their extra buttons/macros/features work. I believe there are a few manufacturers who offer Linux support, but the majority don't. For instance, not even my thumb buttons on my Razer DeathAdder work out of the box in Linux. The easiest solution would be to buy a mouse and keyboard that have official Linux support (I happen to have wanted a new set anyway) but that is not the most economical solution.
I started playing CS:GO on Windows, but I have since moved away from that platform. I play at a relatively high level (LEM) and I had my config and mouse settings PERFECT on Windows. I want to play on Linux, however the tools are not there for me to be an effective player. I assume my mouse is running at the factory 3500 DPI, whereas on Windows I was using 400 DPI. I am looking for a simple gaming mouse with hardware DPI selection...
I could care less about my keyboard macro keys, never used them anyway (aren't useful for the games I play) but I'm sure there are some who do. However I really liked being able to use the thumb buttons on my mouse for PTT in TeamSpeak. There just isn't a better key for that use to me in terms of comfort and not interfering with other system functions.
I digress. What are your specific but overlooked issues with gaming on Linux? Remember, we aren't here to discuss how many games are on Linux, performance, etc...