I have several blogs - Dogs, Paganism, Recipes, clip art - and by far my most popular one is the dogs. I find that the big thing with Blogs is updating them every day or two, using pictures to attract attention and always having quality content. Advertising is also helpful. You can research popular search terms and do a blog on whats most popular right now, but I find its better to do a blog on something you really enjoy as you'll be more enthusiastic about updating it and keeping it fresh and fun.
It varies from person to person and what they are interested in. I like blogs with fashion, funny pictures, personal experiences (the funnier the better), sometimes a poem, cute quotes, song lyrics, animation, links to other interesting websites, etc. Also putting up a cool background & decorating it nicely will attract attention :)
Just something unique and informational, I guess