Valves continued support makes this game long lasting.
I rated the difficulty "Just Right" because it may be hard for some, while easy for others. Some classes are easier to play at start, while others take practice. For me playing as long as I have already, all classes seem easy.
Although I find times where I am bored with the game, the Team Fortress 2 team always does something that brings me back to play some more.
Update: Although Valve continues to update Team Fortress 2, it seems as though the game is losing it's original feel with items that do not fit the game. The most noticeable items are the community items brought into the game. I find that most, if not all, of these items ruin the look of Team Fortress 2. Because of this I have lowered the games rating from 10 to 9.