Having Undead as a playable race is something we're really excited about. We're not just talking about essentially human cities and units with zombie graphics tacked on. We are looking at fundamental differences in the way an entire civilization of undead creatures would operate, and incorporating those differences into our game mechanics.
Undead neither produce nor consume food as most other races do. Instead of food, undead are sustained by negative energy, and instead of farmers, a portion of the population will be allocated to drawing energy from the negative plane. Thus an undead city on the negative plane will thrive, while on other planes undead will be somewhat harder to manage.
Furthermore, undead have no use for nor interest in gold, nor are other races likely to engage in trade with them. As an emperor you may need gold for various purposes, but you will not be able to raise it from the population in the form of taxes or by building trade goods. On the other hand, while individual undead laborers are lethargic at best, they work without need of rest or sleep, and so offer greater production than all but the most industrious races.
Undead ignore all morale effects, both positive and negative. Morale-affecting events simply will not apply to the undead, and buildings which influence morale in other races will have very different functionality in undead cities. For example, temples will spread waves of negative energy throughout the city's surroundings, animating and summoning cadavers, and thus adding population growth.