Well, though some have voted "High", none have stated why. I almost always play high, and these are the reasons:

A) You have an interesting rooster of units, some will be obsolete, and still have a goal of "next generation" units to aim for. (depending on faction)

B) You have the full "rock-sissor-paper" experience, as polearms and crossbows has arrived.

C) There are two gigantic impacts on the game: Mongols and Gunpowder, that both come in High. Both are more challeging and interresting if you havn't prepared for them the last 100 turns.

On a side note, I believe that you will always have pretty much won the game, or at least controlling it before you come to 1205. That is why I always play GA and limit myself to one era - I must win when the 1320 count is made.

When I feel like a crusade where peasants can make a difference, or I want to historically play a crusade (or a defence of your homeland against the crusaders) I play Early.

/KotR