I think it is looking good!
Could you review the following:
The "Stop Sharing" button is actually a button to exit the dialog.
I think we should place this button on the bottom of the window and text it with the label "close".
The text "Click here to share your screen" could be to 10 pixels down and the start/pause screensharing toggle button could be where is now the "stop" (close) button and
And I would slight modify the Tabs:
Could we label the "publish" tab "Advanced" (since nobody has to enter data here)
And in the tab a short description text "Those settings enable you to stream your screen data to 3th party providers like justin.tv"
Let me know what you think of this,