This is a tough one. All the user research I've seen says that more people prefer links to open in the same page. Not everyone of course, but more people on average. In fact, in web design circles opening new windows/tabs is generally thought of as a kind of cheap trick to keep people from leaving your site.
But that's just research and does not apply to everyone or all sites. One might argue that the YLF forum is different and opening links in new windows would be better.
But here is the real issue...
As it is today where external links open in the same window by default, people can control-click (command click on the Mac) to force it to open in a new window. People have the choice. This was mentioned earlier in the thread.
On the other hand, if I *do* make external links open in new windows by default there is no way for a user to override that and make them open in the same window. There is no control-click (command click) option for "same window".
So the only solution would be a per-user setting where you decide the behavior for yourself. This is possible, but tricky because we'd want to treat internal and external links differently. I do actually have this on my list because it has come up before.
BTW, here is one article that says the above in a bit more detail.
http://www.smashingmagazine.co.....w-windows/