The Meteor

The mystical meteor seen in the WWI 2008 intro cinematic is obviously important to Diablo 3's story line, but the purpose of it, or what it is, remains unknown.

Meteor

The infamous meteor sailing towards Tristram Cathedral.

In the Diablo 3 cinematic trailer from WWI 2008, twenty years after the destruction of the Worldstone and the defeat of the Prime Evils, a blazing meteor is seen falling across the sky, landing abruptly upon the New Tristram Cathedral. Leah and Deckard Cain were at the cathedral at the time the meteor struck crashing through many levels down into the basement. It presumably crashed to the bottom of the basement where Diablo was originally defeated in Diablo I.

In the BlizzCon 2008 playable demo, Captain Rumford says a meteor had struck the cathedral and it has awakened evil within. He asks you to investigate. This awakened evil turns out to be King Leoric, or possibly something deep underneath, where the demo characters were not able to proceed to. If this will be the same in the full game is unknown.

Cinematic Meteor Event

This is the sequence in which the cinematic movie from WWI 2008 depicts the meteor:

Meteor across the horizon.
Comes tumbling down...
And down...
New Tristram Cathedral.
Light from meteor shines in.
Crashes through the roof...
Impacts in Tristram Cathedral.
Great bang ensues.
Possibly a view of the crater left by the meteor.
Diablo behind it?

Speculation

What the meteor actually contains is unknown. The only things fans have been able to speculate about are information given in the concept art.

Fallen Angel

The most popular line of thought is that the comet in fact is an Angel, or even an Archangel expelled from the High Heavens for unknown reasons and that hitting Tristram Cathedral is either a sign to get the hero characters of D2 there, or random chance. The reason for this line of belief is the concept art with angel-like appearance. The two most popular candidates are Tyrael and Izual.

These are the concept art pieces that have been speculated to be related:

Was Cain originally supposed to be in the Cathedral, or was he just not visible in the cinematic?
Meteor impacts, but does not hurt Leah.
Presumably Leah looks down the crater.
Some Angel-type entity doing something involving fireing up his wings...
Tyrael on Diablo3.com changed wing-look on Dec 10 2008, is this related to the left image?
Entity with angel-like orange "wings" standing in a bastion of sorts. Wears helmet with horns, or has horns.
Looks a lot like the previous Angel in the tower, has a helmet with horns/wings, and apparently turns to fire/light. Tyrael has no such helmet normally.