If I were to pick one word, it would be "compatibility". Which is not surprising, given the
amount of effort MS has put into enforcing lock-in. It's also captured pretty well in several
of the hurdles mentioned in this article.
Hardware compatibility is one obvious issue. Compatibility with closed protocols/formats is
another (think Flash). Compatibility with widely-used tools is another (MS Word).
If even half of those problems were magically solved, I would have no trouble persuading other
family members to go with Linux. There are other hurdles, but I think that family is the most
significant.