Mostly gun customization and a better ranking system.
The guns sucked. I don't remember the names of any of the guns, just that you might as well use the one AR or the one SMG
(after Googling, were they the Hemlock AR and CAR SMG?). Attachments were lacking. The great thing about COD is that for the most part there's no "it" gun. Yeah, there's always a few that are OP, but that doesn't stop you from using other guns effectively. There are hundreds (maybe thousands) of different variations of weapons in COD that you can fine-tune to your play style, preferences, etc.
As far as the ranks, I'm not even sure if I'm remembering them correctly. But was it just play more - level up more, and once you reached the top level, that was it? Your level wasn't impacted by your skill (a-la Halo), and there was no more progressing after reaching the level cap (something COD's Prestige Mode takes care of).
I could be way off on both of those points, but I played the game a ton for a few months and then never touched it again, and subconsciously those are the reasons I'm coming up with
.