Interesting Damyon - but..
First sentence on that Moodle icons link
Not only are the new icons a little bigger and more colourful
And forgive me for saying, but in those screenshots the background of the buttons is not white - they are grey on grey - so how does that help the contrast for accessibility? I'm not partially sighted, but the icon for highlighting/background colour and the little t and w in the paste icons - well, maybe its the sceenshot quality, but they really are not easy to see (for me).
Right now, for me, one of the first things I change in my theme is those grey icons - I guess I'll have to look into how to do that for TinyMCE as well from now on (edit: Just seen Jasons post above - thanks Jason!). I hope they can be changed directly in the theme, or are we going to have to create a separate TinyMCE theme and install it into the TinyMCE folder somewhere? Consistency doesn't have to mean monochrome! The guidelines you pointed to have a colour palette and style guide which can be used for consistency which would not mean turning everything grey-on-grey!
I do realise that is just my own opinion and maybe goes against what others think - I just hope we have the flexibility to restyle it easily for those like me who want a bit of colour in our lives