Another factor in the wide variety of novels is what is often referred to as the heat level. Want a book to steam up the windows on a cold night? Or are you looking for something you can read in front of your mom (without blushing)? Either way, the romance section of your local store has you covered.
I've heard more than once that steamy books sell, that you need to keep the heat up if you want to make any money. On the other hand, the rise of Amish Romance novels, Harlequin's Heartwarming and Love Inspired books, and digital first imprints like Entangled's Bliss line show that there is growing market for what is often termed "Sweet Romance". The definitions of sweet seems to vary, but these books generally keep most action above the neck, and with either euphemistic sex scenes, closed door bedroom scenes, or none at all.
Personally, I prefer to write fairly "clean", with no graphic sex/violence/swearing. But I certainly read books with all of the above, and think there is room in the market for a variety of heat levels. How about you? Do you prefer sweet, steamy, or all of the above?