Subject that interestSelect topics that interest you and on which you can write comfortably. Also, be passionate about what you write as it will reflect the personal touch in your blog. However, picking topics that interest you doesn't mean that you forget the reader, unless you are writing only for yourself. A popular blog is one that involves the readers as well.
If your topic is too common, try giving it a different angle. Differentiate your blog. Carrying the same thing can make your blog boring. You can also change the format of your blog, like if you always write in article form, you can try bullet points at times.
If you are trying to be informative stay current on news. But, remember don't overload readers with too many facts and figures. Also, stick to word limit (average 250). Don't go overboard.