Who is your audience?
Do you want to write for your friends, your guildies, or The World At Large? Or maybe you simply have the urge to write for yourself, like a diary. Any of these audiences is fine. The reason you need to think about them, though, is that it will help you determine how frequently you should post and their extensiveness. Knowing your intended audience also helps down the road when you start to think about how -- or even if -- you want to promote your blog. I'm greatly oversimplifying here, but in general, the larger you want your audience to be, the more often you should post and the more detailed the posts should be.
What do you like to talk or read about?
Look at the WoW blogs you like. Sometimes they can point you in the direction you want to go. A common saying among writers is that the "personal is universal." So, believe it or not, whatever your quirky interests are, there's likely an audience for it. Some thoughts: gear lists and comparisons, strategy guides, addon reviews, alts, classes, accomplishments, or just a daily diary. If you're a role-player, another great idea is to write in the voice of your character(s). Although I haven't role-played much lately, I do get a kick from time to time out of writing from my toon's voice.
Do you have any particular expertise?
If you are a gear-head about the game, consider writing up guides or analyses. Do you know paladins inside and out? Are your friends always coming to you to help them write macros? Is uber-PvP your M.O.? If you even remotely consider yourself to be good at anything in the game, consider this as a blog topic.
How much time do you have to commit?
Blogging does not have to be a time sink. You are free to write or not write as often as you please. However, if one of the things you want to get out of your blog is World Reknown, then you need to be able to devote the time it will take to get there. Keep in mind that in addition to your WoW playing time, you'll be adding WoW blogging time to your leisure allowance. Don't take up blogging if it requires you to sacrifice other things you love.
Final thoughts on goals
Don't get intimidated by all this goal stuff. You don't need to take a Franklin Covey course or write anything down or search your soul. Just think about some of these topics before you start your blog in order to (1) get your creative juices focused and (2) decide whether you really want to do this or not. There truly is a lot of interest in WoW blogs -- why do you think AOL started WoW Insider? -- so you have that going for you. However, this widespread interest also means that you've also got a lot of competition. Which leads nicely into tomorrow's post topic.