I find that I can get more traffic if I write a blog that focus on one topic. My friend who has a very wide topic coverage told me that the traffic to her blog is pathetic; even after the blog is six months old. Well, I guess a topic specific focus blog can be found more easily by the visitors as they usually rank higher in a search engine listing!

I think it depends on topic to topic. Some persons club up related topics & got very good traffic & some are just struggling up to make money online but i think it also depends on your promotion skills.

well it's not just search engines that make a blog popular .. just write a good post , u'll find people .. else .. hmm depends on ur luck :D

I think even though you have a wide-range topic for your blog, its traffic will mainly depend on its content. If you provide an interesting, unique and useful article to your visitors/readers, this will help your blog to gain more traffic.

No you should change your theme every time.