I've been using Pixelmator for a few months now (GIMP works great in Linux but I've found it's performance in OS X to be rather poor). It's still closed source software like PS (even if it does use open source libraries), but it's fast enough and gets the job done (my needs are really meager).
The reason why GIMP doesn't have more developers is easy: Programmers don't have any itch to scratch at this point and there isn't currently a big market for prosumer image tools (most folks use PS one way or the other, paid or pirated).
If Adobe managed to somehow magically make piracy of their software impossible I think it's likely that you'd see a number of free and non-free products in strong competition with Photoshop Elements.
The reason why GIMP doesn't have more developers is easy: Programmers don't have any itch to scratch at this point and there isn't currently a big market for prosumer image tools (most folks use PS one way or the other, paid or pirated).
If Adobe managed to somehow magically make piracy of their software impossible I think it's likely that you'd see a number of free and non-free products in strong competition with Photoshop Elements.