I had always considered that Linux distributions were disadvantaged compared the commercial OSs because of the software installation procedures. Use case: a user finds some application on a web page they want to try. When the try to install they have to worry about multiple distributions, multiple versions, root access required... My first impression is bye-bye user.
I've been cleaning spyware off a number of peoples PCs recently and have come to realise that for low-tech users this is in fact a big advantage. By being able to pick software from a distribution provided list these users cannot install malicious software. Talking with a middle-aged couple showed there were key applications they wanted (e.g. Google Earth) and they installed other things (some of which were spyware) but wouldn't have been upset if they couldn't. They were used to Windows running slowly (due to the spyware) which meant they were used to a poorly performing computer. I expect this sort of user makes up a large population and would have a positive experience using Linux.
The current distributions only provide a subset of available software. To provide access to the long tail of available software a user-rating/validating system is required. Idea: When a user downloads a file from the Internet the file is checked against a distribution provided list (using URI or hash etc). If the file is not on a white-list they get the option to install anyway and inform the distribution that they have installed. In this way the distribution knows what is being installed allowing a) files to be blacklisted and affected users to be informed and b) the distribution to know which packages they should include in the distribution repository. This of course has some privacy issues but most users can trust their distribution and always include an opt-out option.
Note there is a packaging format called Zero Install that if adopted would make installing simple.