Avi has some thoughts about SSE from the versioning/synching standpoint:

If you look at SSE as a versioning system, there’s one somewhat glaring omission, which is that it doesn’t seem to handle the repeated merge problem that plagues CVS. In an SSE context, that would happen when, for whatever reason, you try to re-sync with the same set of concurrent changes for a second time (possibly because you’re syncing with a third party who got them from the same place you did originally). In the SSE spec as it stands, these would get marked as conflicting a second time (and potentially a third, and a fourth…), even though the user presumably already resolved that same conflict when they first saw the changes.

It's a good (and constructive) critique - you should read the whole thing if XML formats are of interest to you. My concerns are more pedestrian - in looking at the spec, I see a two duplications between SSE elements and core RSS elements that concern me:

• link: "A required, URL attribute. The URL for related feeds". Ok, how and why is that different from the main (channel) link element?
• id: "A required, string attribute. This is the identifier for the item. The ID MUST be globally unique within the feed and it MUST be identical across feeds if an item is being shared or replicated as part of multiple distinct independent feeds.". Hmm - sounds an awful lot like RSS 2.0's GUID to me. This should be loads of fun in aggregated feeds when they differ.

Overall, it looks ok. Those two duplicates are a bit worrisome though.