Hey y'all! I posted this in a conversation re: differences between instances; I just thought I'd go ahead and make it its own post from the perspective of a user - there's other technical differences for instance admins, but this is an end-user perspective.
So, different instances might have:
Different features (different character counts that may be higher than 500; emote reactions/favourites, different themes, markdown formatting for posts). From experience, different features from different instances have varying compatibility - i.e. Mastodon user favouriting Misskey/Firefish post shows as ❤️ reaction, Misskey user reacting to (vanilla) Mastodon posts counts on Mastodon as a favourite ⭐, Glitch-Soc Mastodon is inter-operable with both.
They may have a specific topic in mind. For example, hachyderm.io and tech.lgbt are tech and LGBTQ+ related; urusai.social and sakurajima.moe (my home instance 😉) has a focus on Japanese media discussion; mastodon.art is dedicated to art; kolektiva.social is political, anti-colonialist, left-wing.
Moderation and rules may differ. Some instances might have bigger moderation staff and/or have small userbase to moderate, and some may have different or additional rules. Some may be more strict on stuff (prohibiting AI unless tagged, no NFTs, stricter NSFW culture, etc), some may ban certain instances for whatever reason (mastodon.art is infamous - for better or for worse - for having strict defederating policies with the rationale of protecting their users). You can choose an instance you're comfortable with and makes you feel safe.
An extension of the previous point; the level of federation between different instances are different too, depending on what they're comfortable with handling. Instances can be completely blocked off, or they can be "silenced" and not appear on federated timelines (but can have people from different instances interact if they follow each other), have media rejected but other posts allowed through.
That's what I can think of off the top of my head when it comes to the differences between mastodon.social and other instances.
One more thing, DO NOT FORGET - this is a FEDERATED social network, that's why we call it the #Fediverse.
For example, if you have someone you know on mastodon.social or firefish.social or misskey.io; you WILL Be able to talk with them (unless your instance has completely blocked those instances). You may have to jump through a few hoops (putting the person's handle suffixed with @mastodon.social, or pasting a URL into the search box), or use extensions like FediAct (https://github.com/Lartsch/FediAct); but barring any restrictions or defederation, you shouldn't have any issues.
APOLOGIES FOR ANY ERRORS THAT I'VE MADE IN THIS POST! I'll give corrections when necessary!
