- The indentation is a must keep. That alone improves the parse-ability a lot.
- The shading is also an improvement.
Now, regarding the buttons to expand the panels…
Let’s consider two aspects: semantics and design.
Semantics
Semantic-wise, the best is the +/- buttons. They clearly indicate the operation they perform: Display more / Display less.
The arrows are somewhat less clear but also acceptable because they are also used with the same semantic meaning in the UI of other software (e.g. Drupal’s collapsible form items).
The worst is the proposed round button. I personally don’t know of any other UI using such buttons with this semantic meaning. You may point out some to me, if you know any. My guess however is that it will confuse new users more than anything else. What I find interesting in m9105826’s proposal is the possibility to have the title part of the collapsed area. I’m not sure it’s the best choice, but it may be considered (see comment #39, page 2: http://www.blenderartists.org/forum/showthread.php?298335-Properties-Panels&p=2410343&viewfull=1#post2410343 )
Design
Assuming that the +/- buttons are accepted as the best semantic choice, we may now consider their design.
A recurring theme of this thread is to unclutter (en?) the UI and get rid of the visual noise: gone are the grip areas, the divider lines and it was even proposed to remove the expand buttons altogether (in which case the question of design would be moot!)
Thus, the +/- buttons should have a subdued design so that there presence is discreet instead of standing out like a sore.
The problem in the early mockups (page 1) is that they presented black (very dark) + buttons instead of the more greyish arrows. They stood out more than the arrows did. My guess is that people who commented against the +/- buttons spoke unconsciously more against the design than against their semantic appropriateness.
At the very least, the +/- buttons shouldn’t stand out more than the current arrows; if I read this thread correctly, they should be even more subdued!
Personally, I see them a small bump into the background colour casting a discreet grey shadow, just enough so that we can guess their presence for when we actually need them, but not too much so that we can overlook them and forget about them when focussing on the actual settings presented inside the panels.
So… Semantics and design… A poor design does not invalidate the most appropriate semantic.