Because it uses an original codebase designed specifically to implement the D&D rules, it's much closer to the rules than Diku derivatives.
ForgottenMUD has a custom MUSHclient interface that displays static pictures of the rooms. It is cosmetically similar to Nightmist. It doesn't have support for other clients.
The game is based on a planar hub with a portal opening to several worlds. On top of its unique areas, the Forgotten Realms and Dragonlance areas from d20MUD have been converted. It also has a nearly infinite wilderness grid.