Brevity is a major one; a lot of arc code is shorter or cleaner than equivalent scheme or CL code. Fewer parenthesis, more logical names, a few bits of syntactic sugar that go a long way, etc. Just compare the code in ac.scm to arc.arc, and decide which you would rather write, or read. (not quite fair, since ac.scm is mostly compiler stuff, and arc.arc is just the basic library of functions, but there is still a noticeable difference in readability and brevity)

I also like arc because it's really easy to change to be the way I want it. Since the whole language definition is less than 2500 lines long, half of that in arc code, and very few functions more than 10 lines long, it's easy to figure out how it works, and easy to change.

Libraries aren't really a strong point of arc, but using Anarki (the community version on github) it's pretty easy to just use PLT's libraries, which are quite extensive. Speed is the only other real "con" to using arc, and I haven't really found it to be much of an issue.

As for tutorials, PG wrote an arc tutorial that is on the main arclanguage.com page. Other than that I don't know of much that has been written specifically about arc. arcfn.com has a lot of nice tips and a decent documentation section, though there isn't a link to that on his front page.

Other than that, SICP and On Lisp will go a long way towards understanding arc. If you have any questions about the differences between arc and CL or scheme, or just questions in general, feel free to ask.

Most of us here are somewhat disappointed by the current lack of activity on the arc forum, and would gladly welcome and answer whatever questions you may have ;)