The Medic
Medics can make or break a team. Their primary purpose is to heal teammates (yeah they can do other stuff but their call in life is to heal people). With their medpack, Medics can quickly repair damage on teammates, and even give them health beyond their base maximum (which dissipates like Quake1 megahealth). Medics can also use their medpacks to heal concussion, tranquilizer darts, gas, and put out fires on pyro victims. If a Medic uses the medpack on an enemy, the enemy will be "infected," and their health will slowly drain down until they die. If a player ever comes in contact with any other player, he will spread the infection to them, and they too will die. The only way to cure an infection is for a friendly Medic to use a medpack on you. Medics use a beefed up version of the MP5, which acts just like the Quake1 Supernail gun (which is stronger than the scout's gun). Medics carry normal and concussion grenades. Unfortunately, since Medics are meant for support, they can only carry 100 points of yellow armor and 90 health.
What's New?
A techie, Half-Life medpack now replaces the Medic's Axe. It's interesting to see the HL health kit up close, and somewhat resembles a large Star Trek Tricorder. This is a bit more realistic than hitting people with an axe, but I always like the campyness of slashing someone with an axe to heal them. The weird thing is that you use the same medpack to infect enemies. Now just imagine this in real life… a battlefield medic running after an enemy with a large pack trying to infect them… food for thought.
The range of healing/infecting is also lessened from TF1. I think this has something to do with the placement and size of the bounding boxes compared to the Half-Life FOV - everything seems set back a little bit. If Quake1 was where you eyes would normally be, in TFC your eyes start at the back of your head. This has the overall effect of forcing you to get very close to someone to heal/infect them, which is much harder with the connectivity issues of Half-Life.
The super nail gun is replaced with the MP5 model, but it works just like the super nail gun of old. It's behavior even predates that of the current version of TF1 (2.9). TFC uses the same number of nails that the original Quake1 super nail gun uses. Unfortunately like everything in TFC, the nails move slower than in Quake1.
Another nice feature is when someone yells "Medic!" a small red cross appears above thier head. This makes it a lot easier for medics to tell who needs health in a crowded room. Only medics on the same team can see this.