As a single player add-on, Play the World just makes additions that are functional, but pretty minor. The expansion's new victory conditions are actually a decent addition to they game, since they allow for shorter games with more focused goals. The additional tribes are ultimately minor, although some of them pack a real punch. The Celts are a powerful early-game force and the Spanish can quickly project military power across difficult terrain with their conquistadors. New terrain modifications include outposts, which are ultimately too expensive to be useful. They come in handy during the early stages of the game, but this is when workers are far too precious to be traded in for the improved visibility afforded by an outpost. However, the new radar stations that can be built later in the game add a helpful defensive bonus. With judiciously placed fortresses, these allow for some nicely fortified borders. Air bases are an inexpensive way to project airpower and prove very useful for offensive operations.
Aside from the new tribes' unique units, there are two new units that are actually just an upgrade path for swordsmen. Swordsmen were a military dead end in Civilization III, where the twentieth century would roll around with packs of these obsolete units still running around. Now they can be upgraded to medieval infantry and then guerillas, which are particularly helpful if you're missing the strategic resources you might need for more powerful armies. The expansion also adds some new interface features which are helpful for sorting discontent cities, moving units in a stack, finding particular units for upgrading, setting rally points where your cities send new units, and automatic bombardment for artillery and aircraft.
The expansion has new buttons for worker actions.
One of the expansion's biggest problems is in the way it treats its included user-made content. Originally, Civilization III shipped with an empty scenario folder. But Play the World has a scenario folder brimming with maps, artwork, scenarios, and fan-made modifications. However, it's unorganized, almost completely undocumented, and sometimes even incomplete. For instance, the epic "Double Your Pleasure" conversion doesn't actually have graphics. Though it's good that the developer added all this user-made content to really help showcase the work of dedicated fans around the world, Play The World presents all this hard work as a messy heap that will probably scare off new players.
Hopefully, Play the World's wretched online performance can be fixed in a patch. If this happens, the Civilization franchise has the potential to be as excellent a multiplayer game as it is a single-player game. Until that time, Play the World is little more than a few pleasant tweaks and a big pile of downloadable content dumped on a CD.