Speaking of lines, there is still some swearing in the game (as expected; this is rated M after all), but notably most of it is in some language other than English (which doesn't really make it much better, but still…), and even then isn't too common unless you use certain voice commands or play a particular class enough. The violence is, as others have said, very cartoony, and most kills just result in ragdolls, though explosions (made by the Soldier or Demoman) and decapitations (also Demoman) will gib appropriately.

As for the community, I must warn you that the game has recently gone free to play (though you'll have to buy something from the MannCo. Store in order to activate certain privileges. Fortunately, they have class starter packs for $0.99; which is a lot cheaper than the $40-50 you would have had to pay otherwise). Consequently, the servers are now rampant with immature noobs that will swear excessively in order to act "cool", and/or are terribly sore losers hat will make a huge fuss when you inevitably dominate them. Fortunately, many servers allow a "!votegag" command for just such an occasion.