Ok, so what it comes down to, is that you don't want escapes to be processed. Wouldn't providing a non-escapable string be far more general, then?
Since '\D+' clashes with quote, maybe /\D+/ is a good choice for the non-escapable string syntax. Only problem is that using it in other places might trigger some reactions as the slashes make everybody think of it as "regex syntax".
Escaping isn't the only thing. Duck typing is also a good reason to differentiate regular expressions and strings. foo.gsub("()", "nil") is distinct from foo.gsub(/()/, "nil"), and both are useful enough to make both usable. There are lots of similar issues - for instance, it would be very useful to make (/foo/ str) return some sort of match data, but that wouldn't be possible if regexps and strings were the same type.
Now we're getting somewhere :) For this argument to really convince me, though, Arc needs better support for user defined types. It should be possible to write special cases of existing functions without touching the core definition. Some core functions use case forms or similar to treat data types differently. Extending those is not really supported. PG has said a couple of times;
"We believe Lisp should let you define new types that are treated just like the built-in types-- just as it lets you define new functions that are treated just like the built-in functions."
Using annotate and rep doesn't feel "just like built-in types" quite yet.