Thought parkin'. Most of the photo annotations I've seen are pretty banal. And it doesn't appear that many of the photos on Flickr take advantage of this capability. I'm willing to bet a trivial percentage of Flickr photos are actually annotated. Annotation is work. Most photos probably speak as a unit. What's the point?
Two options. One: annotating becomes cooler, easier, more interactive, more networked, more weblike. Two: photos automatically generate a small set of annotations, which people can easily filter and add to.