How to select a compelling blog topic

In general when choosing a blog topic it should pass the following criteria:

interesting - entertaining and will reflect positively on people who share (e.g. the 3 things you probably don't know about how to live longer and be happier)
useful - helpful info that will make the person sharing look good (e.g. 5 tips on how do do X better)
awe inspiring (the "wow how cool is that", or "I never would have thought of that" factor) - topics about innovation & discovery that inspire wonder and amazement (e.g. scientific or technical discoveries, images that are visually stunning, people overcoming seemingly insurmountable obstacles). Think of the video of a frumpy, nervous housewife when she went on Britain's Got Talent. The judges were literally making fun of her, until she opened her mouth. The story of Susan Boyle is now legendary. That powerful emotion we all felt created one of the most highly shared videos of all time.

The latter factor has actually been shown by research to be the most impactful in getting attention, 30% more in fact. Tougher to create of course but if what you share takes a new angle to and old problem, you'll be well on your way.