I like this - thanks for the clear explanation!

Known has activities under the hood - the thing that actually gets displayed in your stream is a "post" activity. (If you save a new object but don't post it, it won't get shown.) Other activities, like "like" and "share", are possible, although there's no visual template for them right now.

I definitely agree that objects shouldn't be universally typed. I'm not completely convinced that activities need to be either - I might want to have a "grok" activity, for example, when I go back and mark that I've really understood a bookmarked post, but I certainly wouldn't want to force everyone else to implement that. Perhaps it's enough to have consensus on a suggested few (like "post")?