The real value add of Mastodon is giving everyone an excuse to finally put something on all the weird domains they've been squatting
PSA: If you are in the web view of a remote instance of some user or toot, you cannot directly reply/boost/fav.
But you can copypaste the URL into your local Mastodon server's "Search" field, then it allows you to interact!
Wasn't obvious to me, so maybe boost this if it helped you. :)