That's the main question, I think. I'm pretty computer literature, work as a web developer/designer/programmer and all that, but I've always been a Windows user. Recently, when it came time to reformat my notebook, I decided to just try out Linux because I was curious. I went with Suse. It installed fine, but it was a pain in the ass to get it to recognize my screen size (1600 x 1050), it refused to see my wifi card, and the touchpad wouldn't work.
Fair, enough, I can deal with all that because it's a notebook after all, the drivers aren't at all standard.
But the actual user experince... well, honestly, yuck. The main thing that made me get rid of it was just how crappy everything looked. Widgets were clunky, interface fonts were either too large or too small, everything was jagged, and the web looked simply terrible. I installed Firebird to see if that'd make browsing a little nicer but no luck. Fonts were huge, tiny, and looked like placeholders instead of something any sort of attention to detail had been put towards.
Then I tried upgrading the software. It came with Open Office 1.0; I wanted 1.1. But it didn't look like it was going to happen until I felt up to compiling my own binaries. If someone as tech savvy as me isn't willing to do that, I can guarantee my parents sure as hell won't be up to it.
End result: I got rid of Linux after a day. It wasn't worth the huge amount of effort required to do anythign with and it was ugly and clunky enough that it got in the way of everyday use.
I realize all of these can be improved and I'm sure in the future they will be. When that happens, maybe I'll give Linux another try. But for now, it isn't anywhere near ready for the average user's desktop.