Can you share some of the stuff they were planning? I wasn't as involved with the PC side of the Halo community at that time, and don't remember the interaction in that community
The tag editor (the lifeblood of the Halo engine) will not allow you to edit or open any tag types outside of the 5-8 that are absolutely necessary to get a basic, static map running.
That means no custom skyboxes, no sounds, no weapons or vehicles, no custom lights and no new entity types.
A few of those things were hacked around with sloppily coded, crash prone apps but in the end a lot of it requires you to mod your game into a state in which it will not be compatable with most other player's builds and is probably against the TOS of live.
They left out the importer necessary to bring in custom models for non static BSP map geometry.
The editor itself is extremely buggy and will crash if you try to edit something that looks like its not off limits but is.
You cannot write your own shaders, you can only use a few of the included shader templates because a lot of them flat out don't work or are missing critical inputs.
Campaign is completely off limits, as is AI.
The workflow is downright stupid because the tore out the hot loading feature so if you change one parameter, be prepared to restart your whole SDK to see the changes and probably crash a few times. (Seriously, just this one feature would have been killer compared to H:CE even)
More crashes here and there and a total lack of support.
And then there is the actuall game itself with GFWL and its serial key limitations, dedicated servers, and stupid ass map requirements.
You still have to have the exact version of the custom map before the server starts the game or you can't join, coupled with a general lack of players and patience, that entirely kills any hope for custom maps.
Oh, and don't even get me started on the documentation, or lack thereof.
So much time wasted tracking down dead ends, a lack of the most basic stuff.
I MEAN, JESUS, THEY FUCKING PUSHED OUT A PATCH THAT BROKE VOICE CHAT. WTF.