Hello. (My apologies for writing in English. If you can, please translate this for the benefit of your community. Thanks!)
A new indentation & threading model has been deployed in today's update. Here's a detailed explanation about this from DannyH, including a comparison with the wikitext indentation habits and traditions (and how confusing those "rules" are for almost all newcomers, especially in large threads), and links to past-discussions, phabricator, and some related ideas.
The short-version (an excerpt) is:
In this new version: If you're replying to the most recent post, then your reply just lines up under the previous message. A two-person back and forth conversation just looks flat, and the visual separation is noted with the user name and timestamp.
If you're specifically replying to a previous post, then your reply creates an indented tangent. If everybody responding on that tangent replies to the last message in that subthread, then it'll stay at the same indentation level. But if someone replies to an older message within the subthread, then that creates a third indentation level. It's set to a maximum of 8 possible indentation levels, and we just stop it there because there's a point where you can't fit a lot of text in each line.
The big idea of the new system is that the indentation should actually mean something. You should be able to tell the difference between a simple conversation and a complicated conversation at a glance, and using indented tangents helps you to spot the places in a conversation where there's a disagreement or a deeper level of detail.
A few editors have said they like it, but it wasn't clear to them how it was meant to work, until they'd read this explanation. Please discuss it here and in the topic linked above, and let the team know what suggestions/requests/concerns/ideas you have. Thank you!