[cloudmike]

They can all pretty much be reduced to a skin on top of an abstract syntax tree, since that's all they really are.

Circuit-based skins work well in the right context. Do a Google Image search for "unreal blueprints" to see many examples of them in use. Blueprints visual scripting probably powers the high-level logic in a video game you or someone you know has enjoyed.

The other popular approach is fill-in-the-blanks, a visualization that looks more like a tree than a graph. The first good version of this I saw was the Warcraft 3 scripting system. It inspired the visual scripting system we built for Tribes, SWAT, and the first Bioshock back in the day.

You can also follow Eve for some modern experiments along these lines: https://github.com/witheve/Eve

[tenfingers]

That's a simplistic view. A grammar might not be a 1:1 mapping to an AST (where the AST is just a reduction of a higher-level construct set). Also, an AST is really a generic term, and can mean anything from a parsed representation of the grammar to the final stage of the output code.

The classical example I give when I ask this question, is how do I represent the concept of an abstract "record" (or structure) in any visual language. How do I convey the mapping from definition to instance of any given construct?

These are though problems to represent in a visually compelling and useful way.

[cloudmike]

I agree, they're very tough problems. My summary was reductionist, and though I offered some examples (since that's what was asked for) I didn't mean to claim visual programming is always compelling and/or generally useful for the masses. I've been down that road before and reached other conclusions, though I'm happy other people are still trying to push the idea further.