I would like to suggest that Highlights can serve a significant function in commenting on others' synths.
Looking at YouTube comments we can see that time is a fundamental dimension of the media being consumed. People often refer to specific points in time in the video, referencing a facial expression or quote.
Similarly, SoundCloud's commenting is all about people commenting on exact moments in a waveform because time is a fundamental axis of the media.
Flickr shows us that space is a critical dimension in commenting on images.
I maintain that allowing users to highlight a portion of a synth or highlight zoomed in portions of an image is critical for an engaging conversation.
Enabling users to point at specific parts of a synth without trying to verbally describe what they're trying to refer to is very important.
Photosynths are spatial experiences through and through and therefore you must build commenting that acknowledges this.