The discussion at Comparison of thread styles has given me an idea for a middle-ground solution that maybe, just maybe could satisfy the needs of both newbies and veterans. Here's a mockup:
It recovers the idea of separate "reply/comment" actions, but now it allows the editor to control whether to create a subthread.
The text "Add notes" is intended to suggest that the comment will have several replies attached at the margin (I'm not sure about the wording, it would require testing). Pressing it would always create a new indentation level, even at the last post.
The explicit "Reply to..." box at the end of each thread or subthread entices the user to keep replies at the same level, though Wikipedia editors can still create as many levels as they wish at any time if that's what they want. The path of least resistance creates a sane threading, but experts can override it; just as simple yet powerful interfaces should behave.
(There are two options for what happens after pressing "Add notes" at (2) in the example. It may work the same as "Reply to 5", or it might create a new separate block of notes below the first one at the same depth level, to keep the same structure of the Dynamic style which is isomorphic to the convention of Talk pages at Mediawiki).
Now tear the design appart... :-)