Some thoughts on Mastodon after the first day:

Mastodon has won a fair number of users on design alone. It's refreshing to see a FOSS project that seems to have been fundamentally designed around the product experience. That's a feat on its own. Many of us have migrated from that certain tweety bird site and compared to that this interface is a breath of fresh air.

That said, a fair bit of what makes Mastodon feel fun and exciting right now can be attributed to it being small. It's really satisfying to watch trends develop in real time on the local feed and go to people's profiles and look at their conversations and see who talks to who. And participating in those conversations feels really rewarding right now because there's a decent chance that anything you say will get some time as the newest post in the timeline.
The number of concurrent users isn't huge right now. If the user base grows, pretty soon those local and federated timelines are going to become very noisy. Finding fresh content will be harder because you can't go and filter feed the global timelines easily as toots fly by faster than you can read.
As it scales up, we're reduced to the tweety bird model, where you basically only ever interact with the timeline of people you're already following, occasionally straying into new territory when someone you already follow mentions somebody you don't already follow. The signal to noise ratio of trying to read the global timelines will just be too low to even be a tenable option.
I'm not sure what the solution is here. Mastodon has this really neat small community vibe to it right now that reminds me of my first steps on to the internet as a wee lad, joining random PHPBB forums and such, in an age before things really centralized to the degree they have now.
I would like to see a feature that allows me to sort people I follow into separate timelines, which gives me the flexibility to follow lots of people but still filter all of the content by topic.
Also note that none of this applies if, instead, Mastodon grows by having more and more federated servers and users join the server they think best fits their interests. This would keep communities relatively small without isolating them. And that lets a single user participate in those different communities while maintaining the same identity, which is pretty neat. A big issue with that right now is the lack of cultural differentiation between Mastodon instances right now.