The main issue with the Subscriptions was that they were global, and would not inform you what changed, just that there were changes. With the increased dynamicness of the web since good ol' Y2K (especially with ads), this model is much less feasible, whereas a DOM-based model is more robust, and allows further automatic data processing.
I never got past small prototypes, so I look forward to Sieve's release since it is basically someone doing my work for me! :)