Considered by many to be the "granddaddy" of Linux distributions, Debian has risen from No. 6 last year to No. 5 today. With a history that dates back to 1993, Debian is one of the most well-tested and bug-free distros available. It's not necessarily as easy to use as Mint and Ubuntu are, but it's a popular choice among longtime Linux fans.
Another interesting change is that Arch has leaped from No. 9 last year all the way up to No. 6 today, and I have a theory as to why that is. Specifically, as more widely popular distros like Ubuntu and Mint begin turning to the new generation of mobile-style, touch-enabled interfaces, I think more experienced power users are increasingly turning to less "shiny" operating systems like Arch and Debian, where there aren't the same slick, highly graphical interfaces to get in their way.
In any case, Arch is not a distribution that targets beginners, but it's a powerful one for those with the necessary know-how.
Though it occupied the No. 5 spot last year, PCLinuxOS has fallen to this year's No. 7 spot in popularity. It is a nice, user-friendly Linux distribution, though, and it offers out-of-the-box support for many popular graphics and sound cards and peripheral devices. A bootable live CD provides an easy-to-use graphical installer, and the default desktop is the lovely KDE.
Here's another big difference between the top 10 popularity lists for this year and last: CentOS didn't even appear on it last year. This year, however, it's at No. 8, while Sabayon--last year's holder of that spot--is no longer within the top 10. In any case, CentOS is a fully compatible rebuild of Red Hat Enterprise Linux that targets enterprise users who don't need full, paid certification and support. It is now one of the most popular Linux distributions for web servers.