Here's one to get the discussion rolling. What I see is a question about the best representation of the system state in an accordion menu. Visibility of system status from Nielsen's 10 usability heuristics. Following are my rationale for each option:
No symbols at all, just colour to indicate the expansion:
State represented by color.
Minimal design and works well for a regular web user (someone who is intelligent enough to know how to navigate on webpage). Though your second example (weblog tools) is not the same thing as the gmail one.
Gmail is using 3 colors, selection=green, main menu item=dark grey, sub menu items=light grey. Whereas, the weblog is just using 2 colors. I feel the 3 color system has a better call to action than the 2 color system.
State represented by chevrons/arrows.
Personally speaking, I prefer the left&down over the right&down and up&down.
The up&down take the most amount of time to understand. Does up mean the header goes up to reveal stuff? Or does content slide up in the header?
The right&down are straight forward to understand (IMO), right points toward the title and down points towards the content from the title bar.
The left&down are my preferred choice. They have the same simplicity of right&down but are not necessarily in your view. You can check the state by glancing at the chevron on the right end, or you can continue reading the titles on the left end.
Plus (+) and minus (-):
State represented by symbols +/-
The same argument as the chevrons, I prefer the right end placement of the +/- symbols since they are present, but in the periphery and not necessarily demand my attention.
One alternative you missed is:
Double enforcement of the state, use color in combination with the chevron/plus-minus symbols.
Now the choice between chevrons, +/- or the bar colors is more of a design decision to me. But, I would first of all try and see if the double enforcement looks usable or not. If not, I would go with the chevrons/plus-minus for a more minimal theme (if you are not using many colors) and go with the color if I want a more color centric minimal design where you have many colors but not many icons/symbols.