UPDATED 2 November 2010; see last paragraph.
This blog, fairly obviously, is published on blogger.com, which is now owned by Google. Blogger.com is geared primarily towards people who want to write but don't want to have to worry about the nitty-gritty technical details involved (such as HTML, CSS and so on). Sign up, click the 'New Post' button, and off you go...almost easier than falling out of your chair.
For the Web wizards among you, you can get into the guts of how your blog and posts are laid out and formatted; there are several in-the-tin and numerous third-party templates that can be used to style your blog any way you like it, and those can then be hand-tweaked by you to get things just so.
As with any click-and-go interface, the Blogger new-post window (what I'm typing in right now) has a "Preview" button (or actually, link); click on it and you'll see a more-or-less-reasonable facsimile of what your breathless prose will look like to the next visitor who stumbles across your blog. I did say "more-or-less-reasonable"...
One thing the preview area doesn't do — sensibly in hindsight — is to apply your template's formatting to the post being previewed. So that, for instance, if your template specifies that you want 9-point Gentium Book Basic, Times New Roman or Times (in that order) for your body, you won't see it that way in the preview.
What you also won't see — and this is what tripped me up for the longest time — is other styling for the post body, particularly justification. You may notice that all my posts are displayed with justified left and right margins; the default, and most commonly used setting, is for a justified left and ragged right margin, which I find unattractive. I've tweaked the template I use several times over the years to try and get the effect I was looking for. Each time, the post-preview showed no changes to the text formatting, and so I undid the change without viewing the entire blog normally. I have been instead hand-wrapping the content of an entire post in a <div>...</div> element pair whose only reason for existence was to set text-align: justify;.
This isn't the first time I've blogged about my learning-experiences-that-shouldn't-have-been, and just to be perfectly clear about this, I'm not trying to dis Blogger.com about this. The preview-while-composing feature is merely to let you see the content of the post you're working on without all the editing framework around it. It won't, and arguably shouldn't try to render that content in its final form. You do, after all, have the ability to go back and edit posts you've already published.
This is, at its heart, a cautionary tale for those of us tasked with making the Web easier to use for a wide variety of users. Be careful especially with interface design. Recognise that, barring explicit cues to the contrary, people's assumptions about How Things Work on your site may have only a nodding acquaintance with your own — but if you have some appropriate description in the right place, users are often happy to adjust. But there has to be a balance, or your 'power users' will feel like they're being stifled. How you achieve that balance is, of course, up to you to find out. Good luck.
UPDATE: Blogger have, since I wrote the original post, changed their preview feature (at least for "Blogger in Draft," their next-gen interface, which I use and recommend.) When you click the Preview button now, Blogger opens a new tab (or window, if you're old-school enough not to use tabs) with the preview of your draft post. This preview is exactly as a visitor to the published post would see it, with the exception of a great big Preview stripe in the upper-left corner. A wonderful improvement; thanks, guys!