The perfect example of an expansion.
Two expansions later, Dark Crusade comes out. Tau and the Necrons are the two new factions. While the former offers an aesthetical change, the Necrons require some new schemes to utilize well.
The world map (yes, there actually is one now) consists of continental provinces which can be observed, occupied, reinforced and attacked, RISK-style. Just when I thought this would be the only source of evolution, I realized that a few maps were shaken-up by changing the all-too-standard "destroy your enemy" win condition into "find 5 lost servitors," or "destroy x number of your enemy's buildings;" a welcome change indeed.
Lastly, the decision to make this a stand-alone expansion was brilliant. While you couldn't play all of the previous empires unless you owned the original game and first expansion, the new empires and low price are enough to entice new prospectors to pick this gem up.
Well done, Relic.