This isn't your grandpappy's Call of Duty
Expansion packs have generally been confused offerings. Usually they give nothing but an extension to the original game design. On those rare occasions that the additions are meaningful, the expansion often ends up being a stand-alone version, as with Homeworld: Cataclysm. In other words, you're actually buying a new game.
Call of Duty: United Offensive is an actual expansion pack in that it requires the original, but it also offers all the goodies you'd expect from a stand-alone expansion. Grey Matter Interactive, the developers, aren't changing the setting and are even bringing back some characters, but the additions and improvements will take quite a while to cover in this article.
Let's start first with what hasn't changed. United Offensive retains all the cinematic Hollywood action movie feel of the original. Your various identities will take on swarms of German troops on several maps, including an exciting aerial battle. The B-17 gunner level adds great variety to the experience and still has us wondering just what the developers did to pull it off with what is essentially a highly modified Quake III Arena engine.
Other modifications to the engine include a brand new particle system. This is best epitomized by the new flamethrower (and yes, it did make it into multiplayer!) However, the particle system looks most spectacular with the new explosion effects. The old yellow marijuana plant explosion was a little cheesy and we're rather glad to see it's been improved upon. Smaller, subtler uses of the particle system include ricochets of rifle and other infantry rounds off tanks and other heavy metal. These produce an impressive shower of sparks.
Smoke has also undergone a huge improvement. It's quite difficult to believe how nice it looks and yet how little effect it had on game performance. The smoke is used liberally throughout the singleplayer campaign and appears constantly in multiplayer. Any time a vehicle gets blown up, the smoke will come billowing out from the ruins - but that's not all. Infantry now carry a handy smoke grenade which serves nicely to help cover movement or even draw the attention of sniper or tanker who likes to shoot from range.