Honestly, I'm not a fan.
All the blank space on the right looks pretty bad to me, just unbalanced and kind of bizarre. That's especially true when nothing else does the same thing - bulb/quote/edit feel a bit orphaned now and going from scanning down the left to flicking my eyes way over to the other side of the screen feels quite unnatural, which seems to be the opposite of your intention. In general, if content isn't going to fill my screen I prefer it in the centre so it's a bit more balanced.
This is also true of several elements of the reply page right now - the embed icons protrude past the elements above and below, the post/preview buttons are floating way out there, and the post history below also still goes across the screen. This screenshot shows what I mean:
While I'm thinking of the most recent posts in that screen, I can't say I'm getting along particularly with the tiled recent replies. Maybe I just need to get used it it, but it's slowed me down following the previous conversation on this thread while writing this. It also means there's a lot of empty space again where there's one long post (like this one) next to a short one that still has to occupy the same vertical space.
Also just a note (probably one you're aware of), the preview doesn't seem to crunch over in the same way the post itself will.