In my opinion, things Flock gets right are:

• A decent theme across platforms (Firefox on Mac could do with a little bit of attention here)
  
• Full-text indexing of visited pages, based on Lucene (Beagle's Firefox extension does this indexing nicely on Linux. I should try Slogger, but it's not working at the moment).
  
• The Shelf for storing snippets (though I'd prefer if it was a bar along the bottom of the screen and had proper previews. For now I'll stick with a cite bookmarklet, which does even better formatting of citations).
  

I already have del.icio.us links handy in Live Bookmarks and via Cocoalicious (using this delicious bookmarklet). Ecto is a great blogging tool and the Flickr web site is pretty social in itself, so that just about covers the rest of the features.