Wicked Wednesday - most popular paranormal romances

I've been checking out lists with the top paranormal romances. What do these books have that make them so popular? I see big authors like J.R. Ward, L.K. Hamilton, Christine Feehan, and Sherrilyn Kenyon. Each of them are different, but they've managed to sell millions.
Here are a few things that I picked out:
1) Every book in the most popular lists is part of a series. There are no stand alone novels. I think it's reasonable to conclude paranormal romance fans love series.
2) A lot of the books feature vampires. Yes, we're told that people are getting tired of vamps, but they still remain the main characters in several best selling paranormal romances. Am I suggesting you write about vampires? No, but if you are, there's still a big fan base for them.
3) Comparing the lists from the last five years, there are more YA books on it now. The YA paranormal romance genre is growing. Readers from all age groups love them.
4) The majority of the books are slow romances. The readers want to experience the build up of tension. Again, this is a change from five years ago where we had more fast romances.
5) The authors have definite formulas. While I enjoy reading books by those big authors, each of their books have similar plots. They've discovered what works for them and they can quickly write new novels.
6) There's still a lot of broad male chests and curvy women on the covers. Not so many couples in an intimate pose, though. I've seen a rise in covers that feature symbols or an object, but the most popular books still have a person or people on them.
What else do you see in popular paranormal romances these days?