Massively's Zenke explained the following on the prototype UI (of which there were several posted): "The character pane shows a health bar and 'mana' -- or Psion, as is referred to in this title... The upper right icons are for basic character functions. The one on the far left would most likely be inventory, represented by the outline of a man.
Moving right, I'd guess that's a 'talent tree' screen for customizing your character, most likely a powers menu (a brain to represent the Psion?), a titles or achievements screen represented by the trophy, and a support ticket system represented by the chat bubble icon.
There are many obvious tips of the hat here to the World of Warcraft interface. Simplicity and streamlined play appear to be the object of the game, but it's interesting to see so many strange icons on the hotbar.
From the choice of abilities, it's easy to surmise that that the game would have been some sort of melee/magic brawler. The upper bar has some similarities to the WoW UI modification Titan Panel - a favorite amongst players."
Also included were a number of in-game level pictures, both concept art and in-level shots, and Gamasutra asked 'Louis Wu', webmaster of the unofficial Halo website Halo.bungie.org to interpret them.
Wu simply noted of this concept screenshot: "None of those [specific] characters or creatures were EVER included in a Halo game."
Of this shot and others in the same series that Gamasutra has not yet printed, Wu commented: "Clearly broken Halo Warthogs, dead marines, a color palette last seen in Halo PC multiplayer levels like 'Infinity'. The presence of an actual Halo in the background of the last shot is also a dead giveaway [as to the game setting]."
Of course, the cancellation of the Ensemble version of a Halo MMO doesn't necessarily preclude other Microsoft-sanctioned developers from working on Halo franchise titles.
And with Microsoft's Phil Spencer recently mentioning: "There are more than two or three teams building Halo things right now", it could be that a Halo title with similarly expansive online options is now being tried at another studio. If so, there has been no public information about it to date.
[UPDATE: The 'Gone Is Gone' weblog has posted a full Flickr gallery of the concept art and UI mockups for the canceled prototype, sourced from the same art portfolio as Gamasutra's original story.]