Thanks to some very eager fellow citizen scientists, I have now tested the new features pretty thoroughly and can summarise the new system as follows: wow this is dogshit.
Now, as to how it works and the specific implementations of the dogshiticity, here is what I have observed from initial experimentation:
OP will get notified of all reblogs, so far as initial testing has gone. This includes tagged reblogs, commented reblogs, and uncommented-untagged "silent" reblogs. This also continues to include reblogs-of-reblogs. The only place OP will be able to see this is in their activity feed, because there is no longer any single location to see all notes/ reblogs on a post. Nobody else will be able to see ALL reblogs.
OP will get notified of any replies directly to their original post. So if you chatter in the notes of the original, OP will see that.
OP will get notified of any likes directly to their original post.
OP will NOT get notified of any likes to commented subsequent reblogs, as those now form a "new" post. Anyone wishing to leave a like should click the like button on OP's section of a reblogged post, otherwise OP will not know.
OP will NOT get notified of any discussion in the notes on comment-reblogs, as those are discussions on a "new" post.
OP will NOT be able to find any specific reblog chain, because for OP the notes on their post will only ever show the direct-from-OP reblogs and never any subsequent ones.
OP will NOT be able to find anything in their notes, and the Activity Feed is not a reliable place to be able to go hunting for other people's previous reblogs.
OP will NOT be able to turn off reblogging on a post, so there is no way to keep a post contained if it takes off in a way you don't want it to.
Every chain effectively becomes a complete silo, unconnected to any other reblog and only barely connected to the original post.
We will NOT be able to scroll through the notes of a post and look for funny tags or interesting comments, because the notes are now wildly fragmented and there is no central reference point. So it will not be possible to look through the notes and see if a hundred other people have already said that thing you were about to say.
OP will NOT be able to do anything to stop a harassment campaign that fills their activity feed, but also will not be able to find any of it in the notes of their post unless the harassers reblog directly from OP every time. This is a feature which will absolutely be used to harass people.
It is also VERY unhelpful that we don't have language to easily disambiguate "comments" (as in, a reblog that someone added something to, thus creating a new post) and "comments" (non-reblgo chatter that happens in the notes). Not that we can reliably see either of those any more.
So! Now you and I are armed with this valuable knowledge about how the new system works, please go to tumblr's feedback form and explain why this is a series of bad problems that you would like them to stop doing.
The feedback form is available here: https://www.tumblr.com/support
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