Connie, I don't know if series are more popular now than in the past... you would know the trends better than I. All I can say is that my first reading material as a child were in series form -- and that's going back fifty years -- and my reading preferences haven't really changed that much over that time period.
As a kid, I remember reading the "Honey Bunch and Norman" series, then the "Bobbsey Twins", then "Nancy Drew" and "Hardy Boys"; then "Perry Mason" (I think there were more than 20 of those); and the "Angelique" series as I got into adolescence, etc. We certainly had good TV shows back then, shows that lasted a long time, but not many channels, and I rather doubt that TV influenced my reading preferences.
I think it might have more to do with my personality... Given the choice of getting to know a lot of people in a shallow way versus one person in an in-depth way, I would always choose the latter.
I should say, though, that I read single novels as well, and whenever I find a book that I really enjoy, I tend to go back to the store and then purchase everything that the author had written previously, and continue to read that author's books as they are released. Authors like Michael Crichton, Umberto Eco, and Arturo Perez-Reverte don't write in series format, but almost everything they write is terrific.
As for JKR, it's hard to guess what she is capable of doing; we've only been able to see one of her work products thus far. She has said, though, that she won't complete anything that she isn't passionate about, and she obviously doesn't need to write for any reason other than her own personal enjoyment, so I'm betting that anything she allows to be published will be good, and will most likely speak to the heart, as I think that's really what she is all about.
