I have run into many little road-blocks when promoting Linux to friends, family and coworkers. But two really stand out as particularly difficult to overcome since they are not particularly rational positions. Primarily, since they haven't heard of it, they don't trust it. No matter how many virtues you point out about something, if someone hasn't heard it mentioned in the newspaper, on TV, by a celebrity, by their friends, etc. they are not going to adopt it. The majority of people are conformists and they feel comfortable when other people around them are doing the same thing they are. MS and Apple are primarily marketing companies that spend a great deal of money making sure their brands are shown on every street corner and making sure no one using their products feels alone. Linux doesn't (yet) have that kind of marketing push behind it to achieve the self-perpetuating critical mass of users familiar with "the brand". You would have to somehow convince them that they are part of the greater Linux community ... even if it is just bringing them to a LUG or pointing out enough popularity statistics or big groups/corporations that are using/supporting/promoting Linux. Secondarily, people tend to value things they pay a lot of money for and they take for granted things they pay nothing for. So if you are going to someone and saying "Throw away that OS+apps you spent a lot of money to buy and do they same thing with this free stuff" you are going to be fighting uphill since they have a vested interest in making use of this thing they spent a lot of money on ... no matter how painful it may be. If you manage to catch them before they pay the MS-tax or after their MS installation has horribly broken or won't run on their old system anymore after updates then you have half a chance to get Linux on their system but you still run into the first roadblock I mentioned. I have found both of these very difficult to surmount since they are psychological hurdles rather than technical ones.
