IndieWeb Building Blocks that Mastodon supports:
- h-card on comment markup in this patch
- full h-feed, h-entry, h-card on user profiles and post permalinks in this patch later fixed in this patch
- rel-me (and URL in profile UI) https://github.com/Gargron/mastodon/issues/121
- h-entry publishing https://github.com/Gargron/mastodon/issues/122
- Subscribe to Atom feeds directly 
- Update: "I do not plan on supporting Atom feeds that don't have Webfinger and Salmon (i.e. non-interactive, non-user feeds)"
(this section is a stub, please expand!)
- ability to create an account / profile with
- bio (with autolinked URLs)
See Mastodon Github Issues:
Resolved or apparently resolved past issues.
One Primary Instance
Resolution: Apparently resolved as there is a growing number of Mastodon instances (and the "primary" instances is now closed to new signups). See:
One danger, that projects like Mastodon face, is when they are used as a service by more than one user, any downtime will affect multiple users. For instance, on the morning of Nov 11, 2016 the mastodon.social instance was down briefly and displayed a generic server error.
In this regard, promoting "mastodon.social" the site in particular as an alternative to Twitter (e.g. see tweets encouraging "join me on mastodon.social") merely encourage users to go from one silo, to a perhaps more open source-based and more open standards supporting silo, yet still dependent on another central org.
As a contrast, if each user were to run their own Mastodon instance, one instance going down would not affect multiple users in this fashion. Second best would be if users who choose not to run on their own servers were instead encouraged get together with someone that does, which again, would limit the exposure of when one instance goes down. But when such users all join a single server (e.g. the mastodon.social site itself) or any platform with similar architecture, they will suffer from this single point of failure in design.
Update (and one way to resolve this!) As of 2017-04-04 (or earlier?) mastodon.social is no longer accepting sign-ups and directing people to sign-up elsewhere: https://mastodon.social/about
"Due to exceptionally high traffic, registrations on this instance are closed until quality of service can be assured for existing users. Choose one of the various other public instances to sign up!"
- GNU social