A problem with Planescape as a MMO is that Sigil is too weird and high powered to be a starting point. To hook players that aren't familiar with the setting you have to start simple. WoW doesn't start in Stormwind, it starts in Elwynn Forest by a little cathedral that's troubled by wolves and kobolds.
You're right, throwing stuff at the player all at once is no way to start out a game.
But Sigil does allow a more gradual introduction, too. Imagine you choose a "home plane" at character creation that doesn't include Sigil: you have your own little newbie area that, after a few intro quests, can open up into Sigil proper. Maybe it's tied to alignment, so:
Lawful Good: You begin in a small coastal village in Lunia where you find lost Lantern archons and assist aspirant paladins and protect evil creatures seeking to reform. Perhaps you meet some vestiges of the Order of the Planes-Militant, or have to thwart a minor attempt at a fiendish invasion by diabolical boat.
Lawful Neutral: You begin in a ordered farming village in Mechanus where Formians help till the fields -- because they technically rule the town. You interact with members of the Guvners, seeking knowledge of how Mechanus works, perhaps fighting the occasional "dark modron" incursion.
Lawful Evil: In a military city ruled by devils you are conscripted to help in a minor scuffle with Tiamat, to find a new portal to be used in the Blood War, and retrieve a lost skull from the Pillar of Skulls, stolen by a prideful lemure intent on rising to the next rank.
...etc.
One of the home planes could even be Sigil, so you could start off fighting rats and being a light-boy for travelers in a small cul-de-sac area of the Lower ward.
