I just bought an 04 WRX. Before buying it I was aware of the issue it has. After going WOT for a few seconds, the throttle seems to stay partially open even with the gas completely released. If you clutch in or put the car in neutral, the RPMs will go up at a rate of about 500RPM per second until redline. The only thing that stops the issue is if you rev then engine a few times. To me this sounds like the throttle (not the gas pedal) is sticking, and revving the engine is the only thing that un-sticks it.
What I'd like to know is if the WRX uses a mechanical throttle linkage or if it's a drive by wire throttle. If it's linkage, then it would make sense that it's simply sticking and may just need some WD40 or a break in. If it's DBW, it would seem I need an ECU re-flash.
I should mention that the car hasn't been driven in a while with the exception of my test drives so if it's a mechanical throttle link then it might make sense that it just needs to be broken in.