Certainly the individual questions and answers should rank highly. They do a somewhat better job at SEO on those pages, although there still is no meta description tag, leaving Google to guess what to put on the search results page.

They could do a lot better on those pages too. The real content is buried below lots of javascript and other code. The higher on the page your real content is, the better. They do have pretty good titles and H1 tags on those pages, and the urls are okay, although I would move the name of the article up at least a level. Here is one: /questions/15181744/twitter-number-of-tweets-not-updating-testing-on-local

But the original topic here was a search for "stack overflow" and for that search there could be no better page than the Home page (well, maybe the About page, although if they did their job better they would get one of those results with lots of sub-section links). There was no real attempt to optimize that page for search engines. Optimizing it for the keyword "stack overflow" isn't going to hurt the rank of those question/answer pages.