Expand/collapse is one option of course. I won't argue against it, but would like to point out some consequences.

First, when you open a thread, would you like to read that as prose, or do you prefer to browse a windows registry? (Sigh.)

Secondly, the way you read a thread will have an effect on how you contribute. A considerate monk might decide against expanding on a topic that is already large and sprawling. A collapsible tree might work differently. IOW, there is another objective that is at odds with the ease of browsing. That objective is to keep a topic in focus, topical, limited, and manageable.

Not sure what the value of letting users reorder nodes at will would be. But perhaps, some reply nodes could be designated as subroots and appear in main sections. Almost like moving, but without breaking the tree relationship. (A placeholder hyperlink would remain.)