I was kinda assuming that the system already has a separate appserver tier from the webserver tier (HN doesn't, but many other real-world apps would). Separating application logic from HTML formatting is usually a good idea, if only because HTML templating tends to be CPU-intensive but memory-light while application logic is often CPU-light but memory-intensive. That's an orthogonal issue, though - you can run as many appserver instances as are necessary for your dataset.

I guess I was wondering why, in your app server, you don't just add a big in-process heap and use the normal language mechanisms to access it wherever you'd return your leaderboard info?

The concurrency issue is interesting - how does Redis handle it? Does it have some sort of STM, or is it all because everything executes in a single thread in Redis? If it's the latter, you'd get that for free in a single-threaded appserver (although you probably don't want a single-threaded appserver).