This is what my view of Stack Overflow looks like right now:
This is Chrome maximized on my laptop. Not much whitespace to work with at all.
Even if there were more space, I hesitate to put more and more information up. The focus of the page should be the questions. Add too much more on either side, and we'll end up too much like those other Q&A sites where it's hard to tell what is even happening.
As far as creating widgets goes, I don't hate the idea, but I also kinda like the relatively clean look we have now. On some devices I run with a higher resolution and more whitespace. On others, I'm really cramped for space. I think adding widgets would introduce more complexity and ultimately worse UX to the point where few would install them, in which case why are we even building them. :)
Granted, that's speculation. But considering how much work would have to go into adjusting the site design, defining which widgets would exist, building UI for managing them, etc. I'd like to be more sure that it's probably a good idea before we even start.