Among the most notable changes in the update are new tools aimed at preventing transaction malleability attacks, the vulnerability blamed for the loss of user funds at Mt.Gox starting in 2011. While transaction malleability is a feature, rather than a bug, in the Bitcoin Core code, until the latest release coders needed to implement custom protections to their projects to check against transaction ID (TXID) changes.
Other notable changes include the dramatic drop in default transaction fees. The new default setting is 0.01mBTC per kilobyte, about a tenth of the previous fee. This new default setting was implemented to account for the rise in bitcoin price over the last year. Developers noted in the release notes that this change does not force miners to accept low-fee transactions.
Note that getting a transaction relayed across the network does NOT guarantee that the transaction will be accepted by a miner; by default, miners fill their blocks with 50 kilobytes of high-priority transactions, and then with 700 kilobytes of the highest-fee-per-kilobyte transactions.
Other changes include: the release of a Windows 64-bit version of the client; the end of OSX 10.5 and 32-bit support for older Macs; a new block chain checkpoint at block 279,000; support for payment requests; and P2P error messages (useful for explaining transaction failure to users).