Gala is the name of the WM, its compositing features are based on libmutter, but I believe it can run without them, although I haven't tested it. But if not - it's Linux, you can just swap Gala for whatever you wish.
Still, compositing in Pantheon/Gala is done very conservatively, nothing flashy or heavyweight like in Compiz, just your basic fade-in/fade-out and smooth animations, even super old integrated Intel GPUs with no memory of their own can handle it.
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I checked, it does run well without OpenGL.
Oh, and its Wayland support is pretty advanced (official support slated for the next release), should Pyra ever wish to go this way.
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Correction; it does run well without 3D acceleration - Gala still requires OpenGL. Nonetheless, Gala doesn't have any special features besides compositing, one could easily make a similar theme for Openbox and run Pantheon/Openbox, or any other WM.
Another good thing about Pantheon is that it's very intuitive, OSX-like. I installed eOS for all my friends and relatives with old machines, who were frustrated with how slow they ran, but weren't technically inclined to use "real" Linux. I didn't even have to explain where is what, it's that self-explanatory.
As for power users, it doesn't hide anything either, there's even a nice 3rd party app for changing advanced settings of the DE. I'd put Pantheon/eOS somewhere between XFCE, Gnome 3 and Unity, with usability, integration, and performance being the main ideas behind it.