Hey I'm pretty new to this community, I'm one of the Mudlet developers (windows/macos/linux native client).
The conversation touched the topic of UIs and I thought I'd pitch in with my thoughts on this - the power of MU*'s is, obviously, in their text - but text is a steep learning curve to get into. You have to admit that few people in the world like a wall of text, reading instructions, or not getting instant gratification these days.
This is where having a cool UI for your game can help you out. It gives instant gratification to the newbie to see colourful pictures and a cool interface, and it makes them stick around longer to read the reams of text they're gonna have to learn to love. We encourage games and players to create UIs for this very reason, see some sample UIs here. Many other game admins have arrived at the same conclusion - see custom clients for BatMUD, IREs, and so on.
On the topic of server <-> client interaction - I agree that an out of band protocol is necessary for a modern server as it just cuts a lot of the boilerplate out, so to speak, that players will be otherwise forced to create. My preference is GMCP since it is JSON-based and JSON is a lot easier to work with, but we support a few in Mudlet and even have the ability for scripts to add support for more.