Agreeing with a post mentioned above, I am proud to say that Team Fortress Two has one of the best communities around!
Quite a few reasons come to mind, the first being the overall acceptance of new players.
New players are known to get a starter hat called a 'Gibus' and new players are often referenced that way. However, pretty much everyone holds on to thier Gibus fondly, because it reminds them of when they first started playing. Many times players with Gibus are helped out in and out of game by experienced players, and donate items to new players who may not have much.
That is another reason the TF2 community is so great: the generosity. Communities are known to hold giveaways, and players often receive hats and items for free.
New players are always welcome, and older players will do anything to make you a better 'mercenary'!