Further thoughts, after a few days of experience:
Mastodon feels a little more IRC-like than Twitter, and I think there are two independent factors contributing to this:
1.) Not all the content I see is selected by the social graph; there's also the local/federated timeline, which is closer to the topic-centric model (where instances are the analogues of channels or newsgroups or subreddits)
2.) Mastodon is more 'nowish' than Twitter in some ways, despite the lag.
- Similarly, I have no good way to link to a particular toot in a way I could mention out of band, and then take that link and view it in such a way I can easily interact with it.
- No lists, so no lower-volume views of subsets of the social graph.
- DMs are another selective/lower-volume view. Putting them in-band with other notifications makes them get lost more easily. A long-standing group DM has mostly moved back to Twitter for the moment.