Sounds interesting.
One observation, though:
It would always work right on level 2, as they are necessarily replies to the original post. But after that level, there can be different lines of levels, take for example:
1 (starting post)
--2
---3
----4
----4
----4
---3
---3
--2
---3
---3
By displaying level 2, they are all replies to subject 1.
By displaying level 3, you end up showing replies to two different posts, and at the end they could be different subjects.
