Support for Real-Time playback in the preview window.
This feature is mainly useful if using AME as a stand alone encoder, which is genuinely something it excels at. This feature is present in Apple Compressor and was in MPEG Streamclip.
Jeff Young commented
With audio playback, too, please. It would be nice to not have to launch Premiere to cut heads and tails from a recording that has pre-roll and post-roll.