New Multiplayer Options

Variable game speed – hosts can choose quick or epic timescales, depending on the amount of time players can commit to a game.

Hot join – join a game in progress by taking over an AI opponent.

Team play – whether playing multiplayer or single player, team play offers a new way of setting locked alliances that result in shared wonder effects, technology, visibility, unit trading and shared territory that delivers a plethora of new strategic and tactical options.

Simultaneous Turns – speeds up multiplayer games by allowing players to take their turns at the same time, rather than waiting in turn for each player to finish their move.

Connectivity – LAN, Internet, and Play By Email allow options to suit multiple play styles.

Unit Promotions

Modding

Diplomatic Game - AIs forming blocs, AIs having loyalties, many interesting factors affecting relations, religious alliances, clever AI diplomatic actions, really takes Civ4 to a new level of Civ.

Religion - Adds to the depth of (especially Single Player) gameplay with more justifiable tech paths from which to choose, races to found religion, increased importance (and dangers) of open borders, and balancing the benefits of state religion (internally and with other civs that share your faith) versus the diplomatic disadvantages (with the civs that have different state religions).

Great People and Resources and Cottages and Building "Up" : We've gone from "Bigger is Always better" to "Bigger is Usually Better but you'd Better be Smart". This is huge!

Greater selection of map types

See how Civ IV stacks up specifically against Civ III in a bevy of categories: