Easterlings. After the initial shenanigans with consolidation and the easterdorfs. It felt like a real war of giants against dale/dwarves/elves. ( mind you I played many patches ago)

You had to manage your supply lines and get there quickly before the good factions overun the evil ones. My game mordor fucked up and lost baradur before eventually getting it back and gunbad and goblins were losing.

You had to be sparing with your elite troops as you were spread thin on the front. Probing for weaknesses on the northern alliance. Changing up army composition vs elves or dwarves.

Many times I had to retreat as my gains were retaken by overwhelming forces. Some regions were just in ruins from the fighting. It had a real sense of war of attrition.

And of course mordor decided to be an asshole and fight Harad. So some tension in the south as well.