For the most part not that different from the original game. In comparison this one is a little more linear and spectacle-focused, so you might consider this a sort of proto-CoD. Yet still it provides more freedom even in its more linear parts thanks to a better level design. I'd say I actually enjoyed this more than the original Call of Duty.

Another slight difference I noticed is that this expansion requires you to be a little more economical with the ammo. I've regularly run out of ammo for one or more guns, having to resort to pistols, which is something that never happened to me in the original game. On one occasion I've run out of ALL ammo and had to melee a few enemies before I found something. I don't know whether this is a good or bad change. On one hand this is less realistic (since every enemy carries a weapon) and can get a bit annoying, on the other hand it forces you to use more weapons and adds more tension.

Unfortunately, Breakthrough carries over all the flaws of the original, though here they are noticeably less egregious. You still have escort missions, but I've only had my AI die on me in one short section of the game (like about 5 min). You still have turret sections and those weird tank/vehicle driving sections, but they're pretty quick and painless. There are still those barely visible snipers in the fog on a few occasions, but this time they're more manageable. I'd say overall this game is much less frustrating. With the exception of the aforementioned escort mission and the last standoff (which was ass), the game is rather painless to play through.

In many ways I'd say this is a superior title to the original, but this will come down to the preference. It definitely places more emphasis on set-pieces and curated experiences, which make you feel like a badass action hero without resorting to the complete on-rails design of Call of Duty. Personally, I prefer the mix of loud and quiet moments that the first game had, with more open levels and occasionally free task-completion order. Though the original game is more frustrating, it also feels more rewarding. There is something about either the pacing or the length that makes this expansion feel precisely like an expansion. Overall though, when comparing it to the original, I'd rank them as equals, despite slight difference in gameplay.

Man, this was frustrating at times! There is this cool thing about Allied Assault that when an enemy dies, he drops his weapon on the ground and even if it's not the same as your weapon, you can still pick up the rounds. Breakthrough, on the other hand, thinks it's more fun if weapons just disappeared when dropped. Well, it's not. It's actually frustrating as hell and there are missions that you just run out of ammo and have to melee attack your way through. Also, for an expansion pack, it felt to me that it was dragging a little bit. Especially the Italy section. But I guess I can forgive that because we don't often get to see the battles that took place in Italy in WWII video games.