Just a few examples of the differences: the Hierarchy build massive battle walkers that double as mobile factories instead of buildings; Novus can convert units into energy form and quickly warp them across the map through a network of cloaked transmitter towers; and the Masari can switch from an offensive Light mode to defensive Dark mode on the fly to suit any given combat situation.
For this reason, multiplayer is the star of the show, with the three sides interacting in unpredictable ways as new strategies emerge. There are some really unexpected twists - for example, if two Novus players allow their networks to interconnect, units can flow freely from one base into another.
Switching from one faction to another means you basically have to learn how to play all over again multiple times throughout the game. For 360 players whose only RTS experience thus far is Command & Conquer 3 or Battle for Middle-earth II, the learning curve might be a turn off. Compounding the intimidation factor are the controls; the interface is as good as any other console RTS to date, but browsing through so many options with so few buttons inevitably takes extra time and causes extra confusion.
Other problems unique to 360, but common to the 360 RTS, surface as well. You don't get too much zoom control even though the maps are absolutely huge. And distinguishing, selecting and commanding individual units on a television set is incredibly difficult sometimes, regardless of whether you have standard or high definition.
If you have the patience and guts to tackle Universe at War: Earth Assualt, however, you'll find plenty to reward you. For hardcore fans, the deep options and completely unique factions will be a revelation. For newbies, it will be a graduation.