Each unit can be upgraded slightly, such as a spearman to an advanced spearman for a slight cost, or can be upgraded with technology through the ages for a high cost. This is to keep someone from losing just because they didn't rush through the technology tree to the last age. This would be totally cool in multiplayer, where I can imagine dominating some newbie by burning down his huts with my mechs while his spears plink uselessly off reinforced armor. Empire Earth includes a priest that at this point can cast volcano and earthquake. The representative illustrated this by casting volcano in the middle of an enemy town, destroying everything in it.
Walls are also important in the early game, with an emphasis on siege weapons and related units.
Like AOE2 you'll be able to build Wonders, but unlike AOE2 the wonders in Empire Earth all will have some purpose. For example, the demo has a time portal you could use to send futuristic troops back in time to change the course of history. This may not make it to the final release but it would be cool Sierra found some way to manage this and keep the game balanced. Empire earth also includes hero units, such as Elvis who can protect from priests and conversions. The single player game is currently planned to includes 30 scenarios over 3 crusades.