I mention in my article on positioning and messaging that one way of formulating a theme for your marketing campaign is to identify your product's biggest weakness and highlight the strength within that weakness.
Since your product can't be all things to everyone, it will have weaknesses. To be really useful and appealing to your target market, your product will inevitably turn off, or at least not appeal to, some people outside your target market. The key is to embrace this weakness and show how your target market benefits from it.
For example, are you marketing a software product that is targeted at power users? If so, ordinary users will probably have a hard time using it. The difficulty results from satisfying the complex needs of the power users. That's okay, however, since power users by definition are able to deal with this complexity. You might therefore choose "power" or "sophistication" as the theme to trumpet to these users.