To be honest, I don't really understand the point of Cap'n Web.. it's a slightly better JSON-RPC I guess?

I'm a fan of Cap'n Proto, but I don't understand how Cap'n Web is in any way related to it nor what makes it a big deal.

The selling point is promise pipelining, being able to do many chained operations with a single api call

This includes the freaky way they found to make this work with map