Ok, let me start off by saying that normally I am not the kind of person who starts using a new piece of software for 5 minutes and then loudly proclaims he (or she) knows much better how it should have been implemented. Normally I try to work quietly to understand the paradigms that are natural for the software at hand and only then offer my thoughts on what might have been done differently (in my opinion).
So, that out of the way, please take the following suggestion as: Here is a thought that crossed my mind that might make a neat interface change/addition - but keeping in mind that I am a complete noob at this program.
As a daily user of Nuke, I have come to really appreciate the parameters panel in that app. It operates very similarly to the parameters in Terragen in that each node, when double clicked, brings up its own small "window" of parameters. The nice thing about Nuke is that these little windows stack up in their own frame on the right side or the interface (or wherever you move it to). They stack up such that the latest one you double clicked sits at the top of the stack. The previous ones push downward in screen space but are still visible (a scroll bar appears if there are more parameter windows than there is vertical space). After a user-defined number of parameter windows are open, the bottom-most one disappears off the stack (I usually like to keep about five open). This is similar to Terragen's "stay open" button always being clicked on by default, but the oldest one of these parameter windows disappearing after there are more than (for example) five of them open. It would let me see and compare multiple nodes at once.
Again, I am super new to Terragen and if this is a lousy suggestion, please just ignore it. I hate the fact that I am that guy right now. But if it is a useful request, then... well there you have it.
Here is a screen shot with three nodes and their parameters listed.