Creator of the Middle Earth map here :-D Just seeing this thread for the first time, and overjoyed to see my map has gotten enough play to produce such a deep discussion of strategy!
I thought I might be able to sketch out the broad strokes, which most experienced players can easily grasp.
1. Gondor is operating against a timer of sorts. As Mordor and Harad pump out 2-hit units and generally mass their armies along the Anduin, they will eventually reach critical mass to capture Minas Tirith and destroy Gondor.
2. Rohan starts the game sandwiched between two enemies, Isengard and Mordor, and can end up having to fight a two-front battle if Isengard isn't crushed early. Gondor generally cannot take on Mordor, Harad and Khand alone, help from Rohan is pretty much required if Gondor is to survive (and vise-versa).
3. The Elves benefit from powerful, highly mobile units, giving them the opportunity to make a decisive intervention in the fate of another player.
4. The Free Peoples suffer from weak units and a remote starting location. Can have a hard time influencing the result of the game.
5-6. Dwarves and Dale start the game poised for a relatively close battle with Rhun. A decisive victory or loss can have ripple effects across the rest of the board. Help from the elves can be the deciding factor in this battle.
7. The Goblins, who start the game poised to capture Lorien, benefit from cheap units, and therefore have the potential to spiral out of control militarily.
8. Saruman starts the game with all his eggs in one basket, and can lose the game from one bad engagement.
9. Mordor starts with a large army and 2-hit nazgul. Mordor has diverse strategic options, but will usually attempt to build up a critical mass to take Minas Tirith. Mordor has the ability to effect strategic outcomes for Rhun, Isengard and the Goblins.
10-11. Khand Harad and Mordor will usually focus all their spite on Gondor. Eventually destroying it if left unchecked.
12. Rhun is about evenly matched with the Dwarves and Dale, but may be outmatched if the Elves pitch in as well. Help from Mordor may make the deciding factor for Rhun.
Deciding factors for the forces of Good:
How well can the Elves mitigate the damage inflicted at Lorien?
Can Gondor's allies reach Minas Tirith in time to save it from destruction?
Does Rohan survive, sandwiched between Isengard and Mordor?
Are the Goblins successfully contained or destroyed?
Deciding factors for the forces of Evil:
Is Isengard able to survive the early game?
Can Gondor be isolated and overwhelmed?
Can Harad mount an effective naval invasion of Gondor, cutting off its income?
How well can Mordor support Rhun and the Goblins?
Hope that's helpful. Thanks for playing Middle Earth!!!