>Mastodon is a reaction to Twitter
I disagree to a large extent - it's nominally Twitteresque...but with a very different audience in mind (mostly hypertechies or artsy folks, so far as I ever saw)
There's also a big push towards self-hosting in the Mastodoniverse/Fediverse (though, admittedly, self-hosting it was extremely painful the last time I tried, and most users don't self-host (though many talk about wanting to))
Global search is an interesting idea ... but it really requires that you be fully federated with every other instance - or that you do it externally through some kind of aggregatable tool like your example of Google across known instances
But when you get into the idea of "global search", you need to define what you mean. Do you mean every toot on every instance that has the hashtag "#hackernews" (on the easy end of the spectrum)?
Do you mean finding the "same" user on every instance on which [s]he has an account (exceptionally difficult - especially since user handles aren't universal (though [almost] universally referencable, there is no guarantee that "@ricklamers" is the same user on every instance)?
One the one hand, being able to find "everything" about a topic could be really interesting
On the other ... some communities/instances are less-federated, more isolated (whether by design or accident), and may not want what they post to be as "findable" as other communities/instances
Maybe that's solvable by making toots visible to the fediverse, instance, or followers only ... but it's definitely something to keep in mind