The question in my mind is "Why don't the extra character(s) justify the semantic implication?"
Yes, having to type ? or ! is an extra character, and over the course of large applications can add up to dozens of extra characters. Still, I'd argue that these extra characters are truly overshadowed by the language semantics that such characters would embody. One of Arc's goals is certainly to be an easy-to-learn Scheme implementation that people can "just use", and semantics of this sort are the kind that generally make the learning process smoother (at least in my own experience).
"Here again we are saved by our axiom that the user is a good programmer. We assume the user doesn't need operators to be called multiple-value-bind or invoke-restart-interactively to remember what they do."
That seems as clear a disagreement with your assertion as any.
Even if we accept this premise, it applies only to the core language and (de facto) standard libraries (for which we can assume familiarity). As soon as the language is defined in therms of it's implementation it also serves as a coding guideline and this is where the problem lies.
The difference between something like `multiple-value-bind` and `is?` -- especially in the given context -- isn't quite comparable to what Paul's speaking about there, I think. Adding an '?' to `is`, and an '!' to `map` is hardly at all comparable to making the function's name hideously long.
