The good and evil sides have been broken down into six factions: Men, Elves and Dwarves versus Mordor, Isengard and Goblins. Each side has its own set of special abilities and a roster of heroes that includes just about everyone from the films (plus a hero creator that lets you write your own Tolkien fiction). You alternate between factions over the course of the good and evil campaigns, so you won't get bored by the same units every mission, yet they're not so different that you have to re-learn how to play every time. By the time you hit multiplayer, you'll know which side best suits your personal play style.
Making things more interesting still, some units have alternate attacks, like the Mountain Giants that can either toss boulders artillery style or punt enemies like footballs at close range. And many heroes can mount and dismount their horses on command, adapting to face different enemies.