While traditional multiplayer sees every class playing a pivotal role in the game, Mann vs. Machine places some weight on certain classes. In particular, success is almost impossible without a Medic and an Engineer. Medics need to be aware of their surroundings at all times and heal teammates quickly, while making sure to upgrade their Medigun or Kritzkrieg between rounds. Engineers' sentry guns can mow down waves upon waves of robots to the point that the robots eventually deploy a sentry buster (a large bomb with legs) to take them down. Dispensers also prove to be invaluable resources, since many players tend to run low on ammo amidst the fracas.
While all of TF2's weapons are balanced to a degree in the game's traditional multiplayer, a number of those weapons may see their usefulness diminished significantly. The Pyro's Backburner, for example, is great for ambushing opponents in multiplayer, but its advantages are reduced during an all-out robotic blitz. The Engineer's Pomsom 6000 ray gun has been called overpowered in the main multiplayer mode, but its buffs are far less useful against the machine menace. And Spies are at an intense disadvantage given the sheer number of targets he needs to cover in order to be effective.
One thing to note is that all weapons can be upgraded in between waves, with upgrades customized to suit each specific weapon. For example, the Soldier can either choose to increase the clip or damage dealt from his rocket launcher or opt to increase the Buff Banner's deployment time. Upgrades are purchased with money picked up from successfully taking out waves or from defeated enemies. In fact, teammates fighting over money drops may be the one instance where they'll fight amongst themselves.
Larger boss bots come in different varieties.
Upgrades are pivotal, as later waves introduce more powerful robots. Larger bosses include a Giant Soldier that only shoots crit rockets, giant Heavies flanked by multiple Medic-bots, and oversized Scouts that bash your brains in with their baseball bat. Another wrinkle includes a colossal tank with an enormous HP bar. Even with Kritz and Buff Banner assistance, the tank can take a lot of punishment and will destroy the base if it makes it that far. Taking out the tank becomes more difficult, considering that the other robots will still be trying to escort the regular bomb to your base, as well.
Team Fortress 2 already has a loyal player base, but Mann vs. Machines has the potential to create an entirely new fanbase. It employs many of the familiar TF2 mechanics that veterans love, while emphasizing a new sense of camaraderie over competition. There are only three maps (Decoy, Coal Town, and Mannworks) for MvM so far, but given how much fun this game mode is to play, I expect that number to rise soon. In the meantime, grab your friends and prepare for the machine uprising.
Don't let them escape with the bomb.