The v3 chathistory support and the always-on[1] multi-client[2] features paired with modern clients (like Goguma) go a long way at providing a modern chat environment. Most others on the server don't even know that they are using IRC.
The built-in websocket support is another key feature for me since it lets me to provide a web client just by serving some static files (I use Gamja for this).
[1] always-on: https://github.com/ergochat/ergo/blob/master/docs/USERGUIDE....
[2] multiclient: https://github.com/ergochat/ergo/blob/master/docs/USERGUIDE....