When you create and post a reply, but you decide (even within a few minutes) to delete it, that’s not allowed.
The post will remain on the forum for 24 hours.
I can understand that after replies to that post haven been created, it would ‘brake’ the thread.
But not allowing that even after a minute, probably even before anybody even has read it?
I’m curious, what is the reasoning behind this?
That’s a discourse thing. I never really thought about it but one thing I could imagine: If you could immediately delete your own posts you could post spam all the time and then delete the posts again. The mods might miss your post, but many of the other people who posted in the same topic before might get your reply as a mail.
If there is a (post withdrawn by author, will be automatically deleted in 24 hours unless flagged)-post mods will likely see it and with one click they can reveal the original content of the post.
Wouldn’t moderators for the specific boards get the mail anyway, and thus be able to react on it?
And if subscribed members got a spam mail, wouldn’t they report it anyway?
I’m curious if this is only motivated by some theoretical fear what might happen, or by concrete experiences that have caused serious problems.
To be honest, I am mentioning this because of some obstacles/limitations that exist for new members when they want to post or reply, and for some other limitations on this forum.
(e.g., you are not allowed to create a reply just saying ‘thank you’)
It feels like the bots are restricting some freedom of operation for humans here…
Thats the reason why you can click on the little heart in the lower right corner of every post, exactly to say “thank you” 
It is allowed, it just doesn’t happen instantly, as you point out yourself.
There are no “board specific moderators” (forget all about what you know from “traditional” forum software 
), and (global) moderators do not necessarily subscribe in e-mail form to all messages.
I don’t receive e-mails—is there a way to report forum spam via the e-mail interface?
This is in the spirit of keeping the discussion tidy:
https://community.metabrainz.org/faq#keep-tidy
For saying “thank you” and nothing else, you can simply use the 
icon button, as that FAQ says and @InvisibleMan78 pointed out.
Discourse is more than just software, it’s also a way of/philosophy for handling discussions, and the software is very much built around these ideas. This leads to things such as posts requiring meaningful content to be allowed (e.g., more than X characters, no "thank you"s), keeping deleted messages around for review (as well as keeping changelogs of edited posts).
The Discourse team wrote a blog going into more depth about their “Universal Rules of Civilized Discourse”, which is an interesting read if you’re into this kind of thing:
The MetaBrainz forums rules haven’t been altered from the default Discourse ones, as they are basically 100% in line with our global Code of Conduct anyway.
Hm, strange things happening anyway.
The post I wanted to delete myself (which is what triggered this question), is still there (because of ‘the 24 hour rule’), but the new one I posted 1 minute later has now been deleted completely. (not by me)
Nothing of much importance got lost, but in case somebody is interested what’s happening, this is the concerning thread: Drums vs. drumset
No, I don’t think so. In most other forums you can send a moderator a PM though…
I love the discourse software. The fact that they don’t use threaded topics and you can read a discussion chonologically and at the same time still know what or who someone is referring to by the use of quotes and direct replies makes things a lot more clear I think.
The “lack” of answers without meaningful content keeps you engaged while reading and for long topics like this there is even a summarize function that only shows you the most interesting posts (based on likes and direct replies).
I also like the concept of badges a lot. It motivates people to participate more and teaches them about features of the forum they haven’t used yet. Maybe some kind of badge system for editors would be nice to?