I've been on a break, so haven't been actively contributing, but it looks like you have all been having fruitful debate.
as the images are customizable i'm ok with whatever is chosen, as our users are really happy with our customization. I'll add my 2c worth into this discussion though in the hope of being somehow useful.
In my view icons should be "intuitive", "simple", "consistent" and clear, which i guess is what this debate is all about.
the icons seem to have run into the hurdle as i mentioned in a nother post about representing "state" vs "action"
what it looks like being debated above is having individual section toggles represent "state" and the "open/close all" toggles represent "action"
Can i suggest implementing different icons for the "open all"/ "close all" and the individual icons to hopefully clear some confusion
if sticking with the arrows for individual toggles, google/windows/moodle all use the arrows to represent state, google/windows tend to have a hover action as well though increasing usability, and as previously mentioned i think windows 7 having the angled triangle instead of the down triangle is better.
I think the power plus icons are ok, a little large, and not quite as simple as i would prefer but that is just my personal preference, i dont like the power/faucet icons as although they look nice are not a "regular" uii type icon to represent open/close, it usually represents "power/turn off"
Here is the screenshot again of our theme, note the different open all / close all icons, i havent put a hover image on the individual "+" icon but i'd like to if i get some time to play with it. Both "individual" and "open/close all" represent action to hopefully be intuitive (as this is how other systems behave), consistent (all action) and clear
Anyhow don't know if i've been helpful or just rambled, hopefully i've at least summed up discussion thus far ;)