Tab Bar - Position
I think the tab bar should be placed only above the right-hand edit pane, not above the file browser sidebar. This would mean that the file browser would be the full-height of the window.
I think this would make more visual sense...
The tabs relate to the files open in the edit pane, so should be attached to this part of the screen, not sneaking out over to the left. Also, when the sidebar is opened/closed the tabs would always be above the edit pane, whatever the state of the sidebar.
Tab Bar - Text Colour
The non-selected tabs are quite hard to read (with black text on a dark grey background). Maybe a lighter text colour would be clearer
File Browser and Gutter - Theme
Please allow the file browser and the gutter (containing the line numbers) to have an optional dark 'theme' - the white/light grey background is quite jarring when you're using a dark theme for the editor
Indentation Level Markers
It would be great to have some indication of the current indent level (to help line things up vertically).
(Sublime Text 2 does this really nicely, with feint, visible lines indicating the indent levels, that brighten in the area that the cursor is positioned in - thus being both subtle and visibly useful when editing an area of text)
Regards
SC