TF2 still doesn't have upgrades or anything else that is similar to Cod4. Cod4 also has other things that make it unique like zombie servers and other mods. I SHAL DEFEND THE PRIDE OF COD4 UNTIL THIS THREAD IS CLOSED!!
Because TF2 doesn't have mods? You sir are sadly misinformed. Well first of all, TF2 has 'sidegrades' not 'upgrades' what are those you ask? Sidegrades are weapons that aren't necessarily any better than the first, but they modify the purpose of the weapon some. Lets take the Medic's Medigun and Kritzkrieg for example, the Medigun can make the user and who they are healing invulnerable for 10 seconds, very useful, the Kritzkrieg, which is unlockable and replaces the Medigun charges 25% faster, and gives the person the medic is healing 100% Critical hits for 10 seconds. Is one necessarily better than the other? no, not really, it depends on your style of play which one you want to use. Anyway, my point is because of this the game is much more beginner friendly, you don't have anyone at super high levels running around with guns that are 10x better than yours. And the lack of grenade spam is also a big plus
The other thing is, every few months TF2 gets a massive new update adding new weapons, new gamemodes, new maps, new unlockables (besides weapons

), and new features, and all that stuff... for free! Sure its only every few months, but it keeps you interested in TF2, it keeps you coming back to have more and more fun

.
And for the mods that TF2 has, well there really are too many to name

, here are some from the top of my head. Rocketpong, Surfing, Pyro Dodgeball, Randomizer, Roll The Dice, A ZOMBIE MOD

, Balloon Race, Prophunt, Hyper Spam, Ammomod, and much much more that I cannot think of at the moment

(its early). Plus, custom skins and models you can use for your weapons to customize how the games look, and the massive amount of community created content for it, the literally thousands of custom maps made for it (granted not all are good

, but there are some that are very, very good).
Plus, Valve listens to the community (more than some give them credit for), and if the community seems to really want a new feature or some such, they will add in that feature, for free!
Also, the 9 classes give a huge opportunity to really choose one and get a playstyle that you like, plus makes teamwork extremely important, aka, there's lots of teamwork (at least in the eGO servers

). But, if you don't like teamwork style of play, choose Spy or Sniper (I'm not saying that they aren't team players, just that you are pretty independent from the rest of your team in your actions as a spy or sniper).
Anyway, all that and its only $20

, for an amazing game, and continuing free updates well past the foreseeable future.