Poll

As someone that has been using Linux about 15+ years, I see the biggest problems like this:

Giving up OSS sound system.
Other *NIX system still uses OSS in one way or another and many programs are able to output sound. First Linux went all Alsa and you had many problems with cracking sound. Playing sound from many sources was nearly impossible so you had to use Arts, Esd etc (Yes, arts and esd was also needed at the time of OSS, but some development could have got rid of that dependency as *BSD did). All the drivers had to be rewritten. To complicate things even more they decided to slap Pu-pu-ppu-ppul-puls audio system to the already broken architecture. OSS was all about what UNIX should be: working with files (simple and elegant). Even to this date, when a Linux user wants Skype to work properly I might have to fight a lot getting the sound to work properly. Many times a growing noise appears which do not appear with other OS.

Broken desktops.
Early we had two competing desktop systems: KDE and Gnome (because of license issue). Every single time I tried Gnome, I had terrible crashes also it was quite sluggish. Opening a folder with pictures took forever as the thumbnails were created each time. It was also lagging terrible behind KDE in many other ways. Still it was the first choice in many distors meaning a new user got a really bad taste of Linux. KDE was still usable, but there was a small chance your distro was running KDE as default.

Then came KDE4 which completely destroyed the only really powerful and usable desktop at that time. Gnome had grown up at that time to be somehow usable. But Gnome also decided to mess up their desktop, which resulted into many new broken desktops got created (Unity, for example).

Now we still have some usable DE like Xfce, but they are still lagging behind what we have and they are not impressing on users coming from other OS.

Many more factors.
I do not intend to write a long article so I just conclude here that there are many more factors preventing Linux to be used in every computer.