I don't deny that News.YC is a real application. OTOH, I've used my Scheme implementation for several commercial projects and I don't think I can port them to the current version of Arc straightforward. So I try to pinpoint the reasons. Libraries can come afterwards, so here I think of the core features.
A couple of things I think indispensable are the ability to write non-consing inner loop and O(1) access structure (vectors)---combining these with macros I can make the speed-critical routine run pretty fast. Although this seems an optimization issue, it is so critical to the extent that, if I weren't have them I couldn't have used Scheme to the projects.
Managing concurrent processes and threads safely and efficiently is another part that the language can help tremendously if it provides a right model. Arc's current choice of atomic-invoke is simple and clean, but questionable in terms of scalability; at least I cannot use that model in my app. (And that model is so fundamental that it'll be pain to change later to go through all atlets and atwiths to replace them for the different primitives.)
Oh BTW, I think the primitive 'dead' in ac.scm line 1083 is missing wrapnil.
