Sure, it may have stopped filming while I was still in diapers, but M*A*S*H is still one of the best shows ever. Here's why:
Great writing. Often hilarious, but not just about the laugh lines. Each character has his or her own distinctive voice, and the main characters (even most of the minor ones too) are fully rendered individuals.
Great acting. The cast really brings the characters to life, not just delivering lines well but getting right the small details of expression and posture that make a scene truly convincing.
Original setting. Who would have thought the Korean war would be a good setting for a comedy? But it works, and the show is ultimately more interesting for its peculiarities.