It is ironic to me that Team Fortress came from a primarily "non-team" oriented period of game development. Plus, it came in the form of a non-commercial mod. Now, Team Fortress 2 comes in with a storm of other professionally developed team-based shooters such as Enemy Territory: Quake Wars. It really shows how the FPS market has changed. Team-based games are far more accessible, as previously shown in my impressions, and that accessibility leads to a great game.
One of the key factors in accessibility is feedback to a player. Any game needs to be able to clearly and efficiently inform a player of what is, has, and will be occuring. After playing the Enemy Territory: Quake Wars demo for a while, I had to bring it's accessibility into question. In ET:QW, it was never very obvious what needed to be done or how players were supposed to go about doing it. Not so in TF2.
Every goal is clearly identified in TF2, and while it may take some players a few tries to learn the maps, it is never tough to understand what the goal is. This is all layered into as few game modes as possible, with as few paths to victory as needed. To put it bluntly; TF2 is simple to understand.
My biggest complaint with ET:QW was the state of confusion I was always in and it is fairly apparent which team-based shooter I am currently playing. TF2 and ET:QW both do many things right, but where TF2 really sells itself is in its communication to the player.
When a player dies in TF2, the feedback is immediate. A quick and clear death camera zooms in and displays the player who took them out. The player instantly receives feedback, that hey, a Sniper can shoot them when they run out into the open. Or that going toe to toe with a Heavy, healed by a Medic, may not be and advisable move in the future.
Another form of feedback in TF2 plays right into the graphical style of the various classes. It is very easy, at first glance, to identify what class a player is and take the appropriate actions. A common example; "Hey that's a Heavy, I better find some cover." It is no different than playing Super Mario Bros and deciding a course of action when confronted by a simple Turtle or an incarnation of Bowser himself. Due to the significant difference in appearance, the reaction is immediate.