Some of my complaints: I never know if something expects a string or a symbol, e.g. keys into the req structure. Symbols sometimes require quotes and sometimes not, interacting confusingly with macros. (To make a hash with key foo and value bar: (obj foo 'bar) - bar needs a quote and foo needs no quote.) Symbols have their own strange syntax, e.g. to define a home page, you use the mysterious (defop || ...) because || is the empty symbol. Using symbols as strings makes the code more confusing; is passwd being used as a variable or as a string-like token? I think it's nice that strings have a consistent syntax: if it's got quotes around it, you know it's a string.

The big advantage of symbols over strings in Lisp is they are interned, so you can compare two symbols for equality in constant time, unlike strings which take O(n) time. But it seems that you could use interned immutable strings.

I agree with you that Arc's marginally mutable strings aren't very useful; being able to only modify individual characters gives you the worst of both worlds.

While I'm complaining about strings, it would be very nice to have something like Python's triple-quote """ that can be used to define a string that contains single and double quotes. To generate my documentation, I'm constantly creating strings that contain double quotes, and it's a big pain to go through and make sure all the backslashes are correct.