This update to the spec is a modest change that's largely a preemptive reaction to SHA1 being broken; large portions of BitTorrent are designed around the 20-byte length of a SHA1 checksum. They've decided to move forward with SHA256 truncated to 20 bytes to avoid incompatibilities with existing infrastructure such as the Mainline DHT.

Beyond the hashing algorithm, some important additions that were previously proposals without widespread use (e.g. merkle tree for hashing pieces) are becoming required. The focus has mostly been on optimizing latency for the P2P protocol and making sane improvements to the file spec. I feel like trackers were largely overlooked in this update, but I'm biased because I work on a popular tracker.

Ideally, BitTorrent would be broken down into separate specifications that could be used together or in separate systems: one for the file format and piece representation for sharing files, one for the P2P protocol, and one for discovery (trackers, DHTs). I want to believe that there would be far more interesting P2P projects if you could just lift robust primitives from BitTorrent.