[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.