The flat icons are just a bit too flat. They look like something someone has drawn up in Microsoft Paint.
I think you've taken 'flat design' a little too literally.
There is too much vertical space on the menu, it makes it look really chunky, not spacious or comfortable.
The contrast between the menu/admin bar and the main content area is just too great. Staring at that screen for too long will be impossible. The menu is far too black and the content area is far too grey and dull. We've got grey at the moment, we need something fresh. Have you seen the TryGhost colour palette? Crisp and fresh, not dark and bulky.
The switch to Open Sans is a good idea, but something about MP6 makes the font look really chunky, not fine and clear.
I like the responsive design, but again why does everything get so big? Particularly the admin bar; I'm not blind and if you are then making it bigger certainly won't help.
Everything seems just a bit too square/sharp. It needs to be soft(er) and easy on the eyes. It can still remain flat.
Not exactly very professional, in my opinion. Not something I'd like to show a client. Yet.