The first decision that you have to make is what to make your blog about--life, love, cats, pie? More importantly, how narrow will your focus be? If you call your blog All Fruit Pies, All the Time! can you still write about cream pies later, if you feel like it? What if one day you want to write about cupcakes? Can you still do that, or will you lose reader interest? Do you care about reader interest?
Range of focus might not seem that important initially, but when push comes to shove the topics you write about are a huge part of your "brand"--that is, how readers recognize and remember you. Thus, this is something every blogger needs to think on from time to time.
The Advantages of DiversityIf you decide to go with the "anything goes" route, you can expect
1)Never to run out of topics. The broader your focus, the more things there are to write about, the less likely you are to get bored or contract cases of the much dreaded writer's block. Saw something funny happen today? Blog about it! Watched a lousy movie? Blog about it! Sat on your ass and ate Doritos? Blog about it!
Eclectic blogs are good for people who have many interests, or the talent and insight to make the mundane engaging.
2)To Reach a Broader Audience. A well written, diverse blog will find more a broader and more diverse audience. You can have the reader who loves herbal teas, the reader looking for book reviews, the reader that likes funny anecdotes, and the reader who wants fashion advice--you can have all of the readers in the world, as long as you've written about something they're interested in at some point. And the good news is, they just might stay. This leads us to--
3) Crossover Appeal. The reader that came to your blog for the book reviews might stay for the cookie recipes and the stories about your pet iguana. They might find that they have other interests in common with you, or they might just like your writing. People like a little variation in their web browsing.
Advantages of The Specialty BlogIf you go for depth rather than breadth,
1) You're More Likely to Know What You're Talking About. If you focus specifically on the one thing you know and love very well, something your already an expert in, you won't have to constantly spend time on research just to write something informative. You'll be less likely to make factual mistakes.
2) You'll Reach an Audience that Shares Your Interest. When you establish yourself as someone that always blogs on a specific subject, the audience that finds you will more than likely be one that likes that topic and wants more of it. This means more engaged readers, which means more comments and followers.
Finding a BalanceMost blogs are ultimately going to fall somewhere in the middle of the spectrum. In my case, I really want to maintain a diverse book blog with a primary focus on urban fantasy and paranormal romance. Adult UF and PNR. Surprisingly, though, it's not always easy.
One of the problems with running an "anything goes" review blog is that it's subject to my personal moods and whims. Consequently, if one month I get in a certain mood and read nothing but young adult novels, my blog starts to look remarkably like a young adult review blog. Then I get more requests for young adult reviews, and if I agree, the cycle continues. I guess what I'm saying is, if you're going to be diverse, you need to be evenly diverse all of the time. For me, that meant planning. I had to make lists, and I had to make schedules.
My advice to other bloggers is: decide what kind of blog you want to be right away, and try to stick to it. That doesn't mean you can't evolve a bit, just don't go back and forth.