(1) when doing the job yourself could get you killed (e.g., roofing or major electrical work);I would add to that list (4) when doing the job incorrectly could result in damage that would cost far more to repair than the original job itself. Other than that, I agree with the recommendations in the article, both general and specific. It explains why we used to change the oil on our old Honda (hard to mess up) but didn't attempt more major repairs (too big a risk of causing serious damage), and why we did all the work on our basement remodel except the wiring (too time-consuming—it would have taken us months to do what the professional did in one day). We have, on the other hand, done nearly all the projects the article lists as "Things You Can Definitely DIY."
(2) when doing the job yourself would take way more time than you're prepared to invest; or
(3) when doing the job yourself would require the purchase of expensive tools that you might never need again.
The little video that accompanies the article is cute, too. I've never had to deal with a broken light bulb, but if I ever do, I'll definitely try the potato trick.