In many ways the first "modern" soft fork, P2SH marked the first time the rules of Bitcoin were changed without the involvement of its creator, Satoshi Nakamoto. 

P2SH is also notable as it: 

• Activated through an advanced polling of miners on the network.
• Helped bring about modern multi-signature wallets, a foundation of Bitcoin sovereignty
• Marked the beginning of the divisions among the principal early developers working on Bitcoin.