Well, two sliders are sometimes not enough though.
Take the ExtractIsocurve command for instance. What I like about it is that you can hover over the surface with the cursor and get a dynamic preview of the isocurves at the current position.
It would be nice to have this kind of functionality in GH.
I'am aware that "nice" is sometimes not enough though.
While I agree that 2D and 3D sliders in GH are a great idea...there are ways to do a 2D slider, even all sorts of custom interfaces through other hoops if needed (as we've already seen with the colour picker method): http://www.livearchitecture.net/archives/4489