@tn5421 
1: Tags allow you to attach hashtags for yourself, it's a discoverability in the search thing.
2: Actually the nsfw tag on mastodon is like that - SensitiveContent came first :)
3: Dunno, I wouldn't, it's a legacy GS feature we keep around in case someone wants it.
4: I have no idea, I've tried to debug that on five seperate occasions and to all appearances it should work properly but it obviously doesn't.
5: Nope, OStatus has no post limit and if you enforce a limit on posts in accepted notices you break federation.  Most instances set it to what's sensible for their own community, but they should accept posts of any length.

That said do be considerate of the fact mastodon smooshes the message into a tiny column if you message someone there.