Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
LXDE seems to be the new hot thing; to cater to users who need a lightweight distribution either out of necessity (older hardware, need to allocate as much memory as possible to applications without giving up a usable DE) or out of preference, pretty much every major distribution has begun to offer an LXDE edition. It's user-friendly but light on resources; it's well-built yet very modular. It just seems like the place to be.
On the other hand, Xfce, previously the DE of choice for lightweight DE enthusiasts, has been the source of these new LXDE users. What do I mean? While some people still do swear by Xfce, it's quickly losing more and more users, and distributions are shifting their development resources away from Xfce (and usually towards LXDE). Why is this? Unlike LXDE, which is consistently getting better with each release, Xfce hasn't really changed in quite a few releases — it has become a sort of static DE. Plus, it just doesn't look as fresh and cutting-edge as the other DEs. (Full disclosure: The only experience I've had with Xfce is with Linux Mint 7 "Gloria" Xfce, and as Ubuntu does to Xubuntu, Linux Mint makes the Xfce version behave a lot more like the GNOME version (as opposed to leaving it with the default Xfce look).) Even looking at DistroWatch statistics (which are alternatively called accurate and inaccurate), Lubuntu has overtaken Xubuntu and even Kubuntu in popularity.