This impacts you because pushes to hg.mozilla.org should now be significantly faster. For example, pushes to mozilla-inbound that used to take 15s now take 2s. Pushes to Try that used to take 45s now take 10s. (Yes, the old replication system really added a lot of overhead.) Pushes to hg.mozilla.org are still not as fast as they could be due to us running the service on 5 year old hardware (we plan to buy new servers this year) and due to the use of NFS on the server. However, I believe push latency is now reasonable for every repo except Try.
The new replication system opens the door to a number of future improvements. We'd like to stand up mirrors in multiple data centers - perhaps even offices - so clients have the fastest connectivity and so we have a better disaster recovery story. The new replication system facilitates this.
The new replication log is effectively a unified pushlog - something people have wanted for years. While there is not yet a public API for it, one could potentially be exposed, perhaps indirectly via Pulse.
It is now trivial for us to stand up new consumers of the replication log that react to repository events merely milliseconds after they occur. This should eventually result in downstream systems like build automation and conversion to Git repos starting and thus completing faster.
Finally, the new replication system has been running unofficially for a few weeks, so you likely won't notice anything different today (other than removal of some printed messages when you push). What changed today is the new system is enabled by default and we have no plans to continue supporting or operating the legacy system. Good riddance.