Although rough edges are showing, this once revolutionary game has held up very well.
Cons: Certain mechanics haven't aged well; Perhaps a little TOO hard on higher difficulties; Solo missions really pale in comparison to other efforts
If I had my way, when listing revolutionary shooters, not only would everyone bring up Half-Life and Halo, they would also mention Call of Duty (and perhaps Medal of Honor Allied Assault if you REALLY count those two group missions as a full on revolution). This game single-handedly brought a more cinematic nature to the genre as a whole, which has since been emulated in more competitors than one can count.
So Call of Duty is important (in case you were wondering why the series was so big in the first place), but alas, it has been almost 7 years since its release and games age. So how does Call of Duty fare?
Pretty damn well actually.
Call of Duty puts you in the boots of three different soldiers on the three different Allied fronts (American, British, Russian) during World War II. There's a bit of carryover between objectives, but for the most part that's about as coherent as the story gets. In this case, that's not a huge problem because you are allowed to move between significant events that actually happened without breaking the fabric of time or becoming some unrealistic Europe-traversing super soldier.
You see, whereas Medal of Honor had attempted to attribute several important actions of WWII to one soldier, Call of Duty insists that you are just one of many disposable grunts. This not only affects the feel of battles, but the way you play them as well. While still an arcade shooter at heart, Call of Duty will cut you to pieces if you don't take cover. And if you want to hit anything with your gun, you better be aiming down the sights (another commonplace device that can be tied to this game). You are also encouraged to flank your opponents and use grenades, although sometimes this is an impossibility.
The good news is that this style of combat has aged well, still feeling very satisfying in this day and age. This is largely due to significant recoil and some very good sound work. You will hear bullets firing through chambers, faint ringing in explosions, battles raging on in the distance, and just about everything you SHOULD hear in a war. Voice acting likewise aids the war feel as you are constantly either being shouted at or whispered to, depending on whether you are trying to sneak or whether a grenade just killed one of your teammates.
However, what really made Call of Duty a standout game in its day and age was the superb scripting. Back when they used to work under the guise of 2015, Infinity Ward made a really well-paced D-Day level in Medal of Honor Allied Assault. This level quickly became the talk of the game, and as you might expect, they significantly ramped up the scripting in Call of Duty.
The result is that the pacing of this game was unlike any other game of its time, and is still pretty strong now. Whether you are riding in the side of a jeep driving through German occupied lines, defending a landing strip from Stuka bomber planes, or helping storm Stalingrad, you get the feeling that, one hair closer and you would have been dead. The game is also really good about shifting up the style and objectives to ensure that you don't get bored in its admittedly short, though dense campaign.
That said, veterans of any subsequent Call of Duties (not counting expansions and spin-offs) may find that a lot of the game lacks the same unique oomph found in later entries. Although some scripting is still strong, most of it is generally just used to spawn more enemies at an inconvenient time. And whereas later Call of Duties had a significant end to missions, the first entry often fades out anticlimactically when your objective is complete.
However, the worst offenders are probably the solo missions where you and at most 2 other people will take out an objective behind enemy lines. Whereas large battles prove to be fun, dynamic, and exciting, these areas (likely a leftover portion of a growing genre) are often overly linear and fairly repetitive. Although they never go on for TOO long, these levels (mostly in the British campaign) push it, and because you are the only target for enemies (who often spawn just around the corner for an ambush), are extremely annoying on higher difficulties.
Actually, another area where Call of Duty has not aged so well is in health and enemy accuracy. While medkits for health and enemies with almost perfect accuracy were normal at the time, that doesn't make either of them particularly fun now. There are enough medkits to ensure that lower difficulties aren't quite as bad, but it's hard to argue with the move to regenerating health when you inevitably find yourself backtracking for 25 more points of health. Health you likely lost from an enemy who shot you through a bush with a machine gun from a couple hundred feet. Oh, and on Veteran mode you don't get any medkits at all. Annoying != Fun.
Once you've finished your whirlwind tour of the campaign, a surprisingly fun multiplayer component awaits you. While Call of Duty 4, this is not, you will find that the core shooting is great (and well balanced to boot) and the maps are still pretty fun. Most modes the series is known for were already added at this point too. It's not a particularly memorable multiplayer component in the long run, but it's fun to turn on for a few rounds every once in a while.
Without a doubt, the biggest enemy to Call of Duty has been age. What was once a brilliant, engaging shooter with a strong multiplayer component is now an unremarkable, ugly shooter with a fairly enjoyable multiplayer component (although to be honest, Call of Duty didn't look that great back in its day). However, despite the rough edges showing, Call of Duty is still very much a great game that is worth your time. Just don't expect quite the level of refinement found in later entries.