When I say that I mean the way it is used currently as a Desktop OS. I've been using Linux a long time from the command line. Every now and then I pop my head up to see what's new in the desktop arena for Linux, but I'm always disappointed.
Here is what I think the problem is...
1. The GUIs are getting slower and slower.
2. Difficult for the GUI to take control of itself if a runaway process is eating up all the CPU time (Task Manger in Windows is much better).
3. Still not enough good hardware support. Configuring simple things like sound and desktop video modes are weak and buggy.
4. Playing movies is always a chore because one "good" media player does not exist for all possible video formats (you have to bounce between MPlayer, VLC, Noatun, KPlayer, etc).
5. Different programs compiled using different libraries (GTK, QT, XLib) all feel and look different. Simple and important features like Cut & Paste and object embedding don't always work between programs.
6. They are no good visual application development tools. A tool similar to Visual Studio is needed in Linux. IDE's like Netbeans and Eclipse are okay for small programs or web scripts, but simply doesn't cut it for full blown desktop application development.
There are more but you get the idea. In my opinion, if some of the above can remedied soon enough and Microsoft is stilling selling their latest OS in the same vein as Vista. Then yes, Linux would have a good chance on the desktop.