Greetings, Andrew, and welcome to Steem! :)

My name is Nathan. I'm one of the two architects (the other being Dan) of the Graphene Blockchain Framework upon which Steem is based. While I am not a Steem developer, I know the Graphene code base better than pretty much anyone except Dan (and perhaps Ben/theoretical, who was instrumental in implementing Graphene).

Being a blockchain framework (not a blockchain itself) Graphene was designed to be highly extensible and customizable; to serve as a platform with which anyone who wants to make a new chain can skip all of the boilerplate (crypto primitives, P2P networking, basic structures like blocks and transactions, etc etc) and get directly to implementing their unique ideas, only having to integrate with a few well-defined points in the code.

Unfortunately, all of this is not particularly well documented, so I welcome your efforts to make it approachable to others. To that end, I'll be happy to make myself available to you as you as you work through the code. If you have any questions or would like any guidance getting familiar with Graphene code, let me know and I'll gladly help out.

Also, to respond to your point about appcoins, I agree in general, but I think you'll find that Steem innovates in several ways far beyond the capabilities of any existing chain. For the most obvious example of this, Steem has no transaction fees, and this has been integral to its success so far, so clearly it could not have been developed on any of the usual blockchains.