When a process is polling bitcoind for new transactions, the existing RPC command listtransactions is not adequate. In particular, since there's no way for listtransactions to report all transaction AFTER a certain point in time, the polling process will necessarily have to grab more and more transactions and sort through them, or risk missing transactions if it uses a limit that's too small.
This new command listtransactionsaftertime solves the issue.
In particular, you use listtransactionsaftertime to get all transactions after a particular time (as timestamped on all transactions). A typical polling process can then grab all transactions after the last seen transaction time and be guaranteed a complete transaction log.
