Well, if that's the criteria, Call of Duty 1's gameplay is pretty different from any of the other games. I imagine the early console versions are too.
As far as SP goes, CoD really hasn't changed. It's all about large set pieces with crappy "AI" buddies that really don't do anything until you hit a certain point, and the events are then scripted out..........
MP has changed, but thanks to the success on consoles, for the worse. Because of that success, MP map design is built for 16 players generally, whereas the first CoD had maps that handled 64 players quite well like Brecourt, and my personal love, Hurtgen
. Doesn't mean I don't like to play 64 player servers in CoD4, it just means the map size and design doesn't really reflect that, a few corridors as opposed to many if you will. I'm also spiteful of CoD being released on the consoles, since it was the last bastion of PC exclusivity (hopefully United Offensive doesn't make the jump) for the CoD games, but at the same time, I'm interested to know if any graphical improvements have been made. Some screens of the box art have character shadows, a feature that would've been nice to see in the real PC game.
CoD was the first game I ever played online back in Summer of 2005. Wasted so many hours to it. Now if only I could find my discs...............