Shelf is people orientated - it makes heavy use of the address book and finds connections between what you're doing now and people you know. Which is fine. But it could be cooler.
Instead of just have a person as an initial seed for the clues how about other things? Starting simply - how about urls?
There's already information out there about urls - for a start there's whether it's owned by someone you know. Or its stats from Alexa. Maybe its PageRank value. Then there's when you last visited it and how often you've visited it and what's changed since then. And whether you del.icio.us-ed it or Duggit or whatever. And whether it was mentioned in any of your RSS or Twitter feeds or emails. You could add notes to annotate it.
The next natural step is your friends - what have they said? Have they added notes? When did they last visit it (ignoring the glaring privacy concerns for the moment)? Where did they go next? Hell, throw it open to everyone. What has the rest of the world got to say about this? Suddenly every page has comments whether they like it or not. And notes and errata. It's a Web! It's a Wiki! It's a Dessert Wax and a Floor Topping!
And then there's places. You're looking at a museum or a gallery and it tells you what pubs and restaurants are nearby. And if any of your friends will be close by. Show you photos from the location. Throw in a map. Maybe some historical information or local trivia. Great for when you're sitting at your desk but even better when you're actually out on the street and you look down at your iBlackickreo95 and it's using Cell location or GPS to work out where you are.
Listening to music? Album covers, lyrics, other albums, recommendations. Films? Stuff from IMDB - the actors, what else they've been in, awards, trivia, more recommendations from my friends.
Nurse! Come quick! I think the restraints are coming loose.