One pain point that exists for activity streams right now is the dispersal of responses over various networks. When I post a blog entry like this one, folks get the opportunity to comment on my blog itself (via TypePad Connect), or they can comment on the copy of my entry that gets sucked into LiveJournal from my Atom feed, or they can comment on the activity that shows up in FriendFeed or Plaxo. If I had it set up, they'd be able to comment on Facebook too.
It would be useful if all of these comments were aggregated together so that the entire thread of conversion could be viewed whatever context you end up reading my entry in. This is, unfortunately, not an easy problem to solve on the decentralized social web. Bloggers are accustomed to being "master of their domain", so in an ideal world they want their blog to be the master source of comments. However, it's clear that FriendFeed users want to leave their comments directly from the FriendFeed UI, not follow the link through to the original entry and comment there.
One idea is to provide some sort of endpoint where comments can be submitted by remote systems, but it's difficult to see how that would work with authenticated comments and with comment forms that have features such as CAPTCHAs. It would also be tricky to get right with the "paste in a chunk of HTML" comment systems such as Disqus and TypePad Connect. Every blog has its own variation of allowed comment markup, too, not to mention odd-ball cases like YouTube's video comments. Coming at it from the other side, it's unlikely that the other systems will be willing to relinquish control of "their" comments; for many sites, the discussions they host are a big part of their value.
Another approach is a more passive model where comments simply get added to an activity stream somewhere and it somehow gets consumed by all other sites displaying comments, but then discovery of these various streams and figuring out how to deal with abuses becomes the problem.
I don't know the solution right now, but I do feel that this is an important problem to work on as we move towards a more decentralized social web; as people start to use more and more different activity aggregators it will become increasingly difficult to stay engaged with the conversations that are going on.