I've talked about this before, but it bears repeating, I think. Of course content is the most important part of a blog post, but as a former typographer I'd like to argue that the appearance of that post plays a big role in how likely it is to be read. You could have the most interesting post in the world, but if it's all in one long, centered paragraph people might skip right over it.
I know most people read blogs in readers these days. It "doesn't matter" what your blog looks like. Yes and no. As far as design, I'd say that maybe that doesn't matter as much, but the format of your posts still largely translates to a reader. Below are 7 tips on improving readability (or the ease of reading text), many of which still apply in a reader.
There's nothing like an overwhelming font to scare off a reader. I don't see it often, but there's still a few using unruly fonts. The "fun fonts" are for titles, headings, etc—not for body text. For max readability, your body text should be simple, not bold, italic, script, etc. Some argue over whether or not it should be serif or sans serif online, but I'm not on either side. Most screens have high enough resolution now to properly display serifs.
2. Left Justified
I know centered might look fancy, but I promise it interferes with readability. We're not used to reading long texts centered (or right justified). For a caption under a photo, sure it works, just not the entire post. The standard for English body text is left justified, and you can get away with full justified if you don't like the jagged edge (though that annoys some people due to the uneven spacing between words).
Not that I'm one to talk, but blog posts shouldn't get too long. And if they do you should take measures to help ease the lengthy text (headings, lists, paragraphing, etc.). Blogs are different from magazines or books in that it's almost more like reading a scroll. It's one long page, and if a reader sees a really long scroll they might skip over it unless that article is particularly useful to them.
I won't recommend word counts or whatever, but for me I look for how far it reaches down my sidebar items. If I hit my labels or archive, I know I'm getting really long. But sometimes if I have a lot of pictures and that doesn't matter as much.
4. Text/Background Color
Not as important for google reader and such, but it still makes a big impact on people visiting your blog. Light background and darker text is just easier to read. I won't tell you to go white background and gray text or anything so rigid, but that contrast is important for max readability.
While books can get away with long paragraphs, blogs can't as much. The audience is different. People reading blogs are looking for a quicker read, and long paragraphs bog that down fast. Your eyes naturally get tired following line after line with no pause—it takes more work to keep track. The more breaks, the easier it is on the eyes (you know, without chopping it up to all one liners).
If you are writing a longer post in particular, it's usually helpful to break up the information under different headings. It's almost like a chapter break, but in shorter form. It helps the reader organize the information you present and makes it less overwhelming.
People love lists. When looking for quick information, lists are the best. Take this post, for example. If someone didn't have time to read everything, they could at least read the list I've presented and get the gist of what I'm saying. If they want to read more under a particular number that might apply to them, then they can. Fast, accessible information for the win.
So there you have it—7 tips guaranteed to improve readability. I know it may not seem important, but you'd be surprised how helpful it is to have a well-formatted blog. Readers already have little time as it is, and you don't want to give them any excuse to skip over you.