Fun, Challenging, Flexible and Unique. This is how I'll summarize my experience with Linux on desktop. Did I do anything useful with it? No, I was tinkering it all of the time to make it work & fit my need.
As an OS Linux beat MS windows but loses out to when it comes to applications. There was never an proper application for mail, Evolution was crap so was Thunderbird. I never liked Thunderbird and always disliked application running on XUL mostly for the fact they don't look native for the same thing I don't use Amarok on Gnome. No proper photo management app till Yorba released ShotWell. Same with media players, until Banshee (I'm ok with mono). OpenOffice works, nothing fancy but annoying on how it does things. AbiWord was better.
Problem for Ubuntu is they have no control of any of these applications or the feature the application offer and their quality. Without quality application OS can't win. Same is happening with Ubuntu, all these years Ubuntu has improved but the application they bundle haven't.
Major hurdle for Ubuntu is the hardwares support especially laptops. Most of the newer laptop model with nvidia Optimus doesn't play well in Ubuntu. Ubuntu can't win users expecting us to hunt working drivers. Ubuntu had taken MS approach to support all known motherboard, CPU & graphics card on planet. I prefer Apple approach of being selective on hardware choices.
Again Ubuntu has no control on hardware manufacturer support for Linux. So had to wait for someone in Linux community to write an drive that works mostly.
With little control on hardware & software it uses, how can Ubuntu win? It can stay alive though.