Asa Dotzler wrote an essay a while back called Linux Not Ready for the Desktop. It was controversial enough (surprise) that Nat asked him to come present at OSCON. Here are his main points:

For regular people to see the value of Linux on the Desktop, it will have to install alongside Windows and bring over all their settings from bookmarks to wallpaper.

API stability is an important story. You shouldn't have to jump through hoops to install packages that you don't get from your distro. On Windows, one Firefox installer installs on every Windows version.

Complexity and choice scare regular people. There are too many distro choices, there are too many desktop choices, there are too many applications, there are too many application settings, etc. Then there's clipboard madness.

Linux must feel comfortable to users. Don't mess with the expectations of Windows users. This includes keyboard shortcuts, button positions, and even th panel position. Its foolish to deviate from what people expect when the value of that deviation isn't high or the cost is.