Just wanted to chime in and say I've noticed this on my 2011 WRX as well. I'm not sure when it started, but I'm pretty sure it wasn't like that when it was brand new (7xxx miles on it right now). Just the "grittiness" feeling described, very slight vibrations only when moving the pedal. At first I thought it was temperature related (only when the engine is cold) but from testing, it turns out that it happens when the engine speed is above 1500 RPM or so. At normal hot idle the pedal is smooth, at cold idle I can feel the roughness, and while driving if I shift at those engine speeds I can feel it, although it's hard to feel over road vibrations. I can reproduce it easily by holding the engine at 1500 RPM in neutral and pumping the clutch.
I've had plenty of cars with hydraulic clutches in the past and never felt anything like this, but the WRX seems to have a fairly convoluted pedal linkage setup and a short hydraulic circuit that I suppose I could see transferring more driveline vibration than some other designs. The fact that it seems to be engine speed related makes me think (hope) it's just driveline vibration, but I'm not sure why it would only be felt when moving the pedal (or why it would be felt through the clutch at all for that matter).
For now I'm assuming this is nothing to worry about -- the car shifts and drives great. I thought about taking it in but I don't really want anything to be taken apart if it doesn't need to be.