There may be more this evening, but for now a few thoughts came up while I was eating lunch. Under the hood, so to speak, the set of parameters for spline-based terrain generation can be quite large. There can easily be tens of different parameters whose exact impacts may not be readily understandable when presented to a casual user in the GUI. The solution I've thought up is presets.
An excellent example of the presets concept, and my immediate source of inspiration, is Filter Forge, a 2D image effects program. The "filters" in Filter Forge are effects that can be applied to an image, and are built upon a node graph and/or code. They can have a large number of parameters. To simplify use a Presets list can be used that sets the parameters to values that are geared toward a particular end.
A similar concept fits the spline-based terrain concept very well. For example, mountain terrains will involve multiple parameters describing the general dimensions (width, height, slope, etc.), noise generation, and erosion. A simple set of presets for different mountain forms would make this much simpler. For example, the Canadian Rockies are more eroded than the more southerly Rocky Mountains, which are rougher than the Appalachians, which are more strongly oriented into parallel ridges than the Catskills. So there could be Canadian Rockies, Rocky Mountains, Appalachian, and Catskill presets.
Anyhow, enough blogging. I finished lunch and need to get back to work.