I can “heart” a post – I presume if enough people do this the post gets promoted somehow. But I don’t see a way to downvote a post, i.e. mark it as irrelevant or repetitive. There’s a “flag” action but I assume that’s for spam or abuse.
PS. Discourse told me the topic “No downvote?” was too short. What kind of micro-management is that?
There is no downvote as far as I know. The flag allows you to mark the post as “off-topic” or “other” (and this allows you to give more insight) but I assume this is not exactly what you are searching for.
Although is different, one interesting functionality is that when the post is very long (now 30+ messages), there is a “summarise this post” button that will make a shorter version with the “most relevant” messages. The likes of the messages impact this as well as other factors.
The minimal length of the titles and the messages is configurable. We are using defaults for now. The reason is to avoid very short titles that are not descriptive (think when there is a lot of people this could be a problem). We can change the limit or remove it.
Oh well.
PS: I don’t think (at least I hope) that “post” is the right word to refer to a thread. To me, “post” typically is synonymous with “message”. You referred to posts of 30+ messages, which confused me momentarily: I’d think a thread could have 30+ messages, but a post would just have one (unless it was quoting that many messages, which I would call over the top :-).
Yup, sorry about that. As you correctly guessed, I was trying to refer to threads of 30+ messages. 
Our (yes, I work for Discourse and am a huge Python fan, hello!) nomenclature is that this whole thing (waves hands) “Is there a downvote operation?” is a topic and each “message” within is a post. We explicitly avoid using the other word because it’s not a threaded discussion.
Don’t want to reopen that debate but I’ll note that people have been calling topics “threads” for decades so we’ll just have to keep explaining it. Preferably without referencing that particular string of posts (which is pretty ridiculous IMO).
Hey, we all have our own hills on which we will die (I’m sure that as developers NONE of you hold strong opinions), “thread” just happens to be one for Jeff. Also, it was a dogfooding experiment on Meta for our “blocked words” feature.
On here, feel free to call them what you like, e.g. topics are “parrots” and closed topics “dead parrots”. 
The single “heart” flag is not enough. The reaction on the message can be different: “I agree with the author”, “I have learned something new”, “Good joke!”, “Thank you for your answer/suggestion”, “The author is a nice person”, “I disagree”, “I have no idea about what is this”. I think Discourse needs more kinds of flags, like on GitHub. I hope this is in plans for future, and currently the Discourse team just gather statistics.
StackOverflow has up, down, and close (with explanation) buttons for questions, up and flag (with explanation) buttons for comments, and up, down, and flag (with explanation) buttons for answers. Down votes are ‘kinder’ than flags, but are often frustrating and unhelpful to the author. “Why the downvote?” is a common author question. I won’t miss a downvote button very much.
Many quick comments such as these are better with a quote.
@ambv @pablogsal It looks like Terry’s account hasn’t been bumped to CPython Core developer yet. Just an FYI, in case of an oversight.
Thank you for the ping! I can see @tjreedy in https://discuss.python.org/groups/committers but I have added him again and made him an owner in case something was not working properly.
Please, tell me if something does still not work properly 
I seem to be able to everything I expect to.