I want to point out a system that existed in one of my favorite shooter games, Battlefield 2142. Now, there's a lot of cross-over between BF 2142 and BF 2, so a lot of this applies to both, but I'll focus on BF 2142.
Battlefield, overall, had a system called the "Comma Rose", which, simply put, was a 'rose' of commands you could pop up on-screen, select an option, and your character would speak it for you(and put it in chat). If I remember correctly, you could hit 'Q' for your player rose, and 'T' for your squad leader/commander rose. Most commonly, people would use their player rose to say things like "Thank you!". In fact, when the rose was removed(I believe in BF3), the community raised a huge stink about not being able to thank people anymore. I want to repeat that: The community was upset about not being able to be gracious to fellow players! I can't remember off the top of my head everything that was on the 'Q' rose, but after using it for a while, you get used to it and for many people using it was second nature.
The 'T' squad leader/commander rose was far more useful. As a squad leader, you could hit 'T' to bring up the rose, and then left click to put a waypoint on the thing you're looking at. Which meant you could give commands to your squad members without taking your hand off your mouse to type, and you could do it while running their too! Commanders could do the same thing with way points, and they also had the ability to use their assets. Their assets included:
- A supply drop(magical ammo crate + vehicle repair box),
- orbital strike(AoE damage),
- EMP(AoE technology disabler),
- SAT-Track(Reveal all enemy units for commander on the entire map - the intent being that the commander relays that information to his team by 'Spotting' them, another feature entirely),
- and UAV(reveal all enemies within a certain radius, and show it to all team members)
T comma rose gave commanders access to all five of those without ever taking their hands off the mouse. I could be piloting a walker(bi-ped armored vehicle) as a commander, drop a supply crate to repair me and offer cover to allies in a street, pop a UAV to reveal nearby enemy positions, and call orbital strikes to kill enemies, prevent movements, etc, and I could do that in mere fractions of a second, while losing almost no ability to pilot the walker I was in. At the same time, I could be issuing commands to my squad leaders, and as soon as they accept the order, it would show up on their squad member's screens and give them a goal to strive towards.
I can't possibly praise this system enough. It's a shame it was swept under the rug without becoming more mainstream across more game genres. You have the ability to communicate between teammates, squad-mates, and even up and down command hierarchies. I could write a book on all of the benefits I gained from using the comma rose in Battlefield - it offered complex rewards and it was so ridiculously simple to use!
In the end, and other answers have touched on this as well, what you want to do is allow players to communicate without just giving them a text chat. Typing text is expensive to any player. They have to take a hand off their mouse, find their bearings on the keyboard, and then move their hand back to the mouse. That is an expensive set of operations in a time-sensitive situation.