I use Flow extensively for reading and replying to feedback about the Content Translation and Compact Language Links projects, and also as my user talk page in the Hebrew Wikipedia.

It happens time and again that people reuse existing topic instead of opening new ones. I'm not sure why—I guess that by definition there must be something in the interface that gets them to do that ;)

Examples:

• https://www.mediawiki.org/w/index.php?title=Topic:T4lta8pdgpwrs04f&topic_showPostId=t70iukvzj3cg7rj3#flow-post-t70iukvzj3cg7rj3 (he literally says: "i thought maybe make a new topic but preferred to resume last one."!)
• https://www.mediawiki.org/wiki/Topic:T4l116q3ruo51ujm (People join the conversation with topics that are rather different from the OP.)

Very long topics become unwieldy. If the whole thread is on the same subject, it is not a big deal, but when the subjects start to diverge, this is a serious problem.

Such things happened with the classic talk pages as well, but for experienced users this was less of a problem, because technically it was fairly easy to refactor the conversation and split the new topic to a separate section. Flow, however, doesn't allow splitting. So for the sake of courtesy I'm forced to reply to the same user.

Solutions I can think of:

• Being able to split some replies to a new topic. This may include creating a topic on another page.
• Some designs changes that discourage people from exploiting an existing topic.
• Automatic archival or "marking as resolved" after one or two days of inactivity.

Of course, if can be something else entirely.