Who's your audience?
Especially during an economic downturn and at a time when print publishers are trying to figure out what's next, conceiving and presenting a book proposal that is likely to garner a large audience could be the difference between being published or not.
I think about this in two ways:
1) Writing for some large group - e.g., teenage girls, people who use the Internet, non-native speakers of English
2) Aggregating a number of groups to make a large group - e.g., word buffs + humor addicts + trivia hounds
Consider audience, too, when choosing a title. If you already had a title, consider whether it needs tweaking with your new ideas about whom you're trying to reach. Since subtitles are popular these days, consider referencing your target groups in the second part of your title.
Additionally, clarifying your audience may change your ideas about which publishers are most likely to have an interest in your book. Recheck your list, if appropriate.