The ability to specify the precise date, hour, minute, and second to begin playback. The scrubber bar is very imprecise.
The ability to specify precise start and end times of a clip to save and then download that clip as fast as possible. This would be useful so that, for example, saving 45 minutes of video doesn’t have to take 45 minutes.
These two features are already listed in the #wishlist:
Also, did you know that you can use a pinch out gesture (two fingers spreading apart) to zoom in the playback timeline for finer control?