In addition, units will take longer to produce and will eventually come to have upkeep costs associated with them--however, they will also have veterancy along the lines of what was introduced in Civ IV. That is, units that survive various skirmishes will eventually grow in power and may be able to select various bonuses to increase their usefulness and survivability.
And as it happens, in Civ V, units may no longer be garrisoned inside your cities, so defending your holdings will have two aspects. One--all cities will automatically defend themselves based on their current growth level and any defensive structures you may have built inside. Two--you'll want to make sure you defend your key cities with army units, possibly building fort structures nearby to enhance your defenses.