Excerpted from my contribution to Blog Design Fatalities by author Ryan Jackson.

One of the most common mistakes I see in blog design are layouts that are image heavy — especially when the images are not
optimized for the web. This can cause extremely slow page loads and
you could lose a visitor if you make them wait too long for your
content. It’s best to use a combination of CSS and small repeating
images whenever possible, and optimize your images with editing
software such as Photoshop.

Another big error I see is sites that are not tested in multiple
browsers and screen resolutions. I test my work in various browsers
on a new Mac running 10.4 with a widescreen Cinema Display, and an
old Dell PC laptop running XP. You can’t always get it perfect in
every browser, especially colors. But at least you can say “Ok, my
site looks great on newer bigger screens, and not too horrible on a
little laptop with a limited color range.” This also allows you to
make sure all of your code renders properly — sometimes you can
a small mistake in your CSS that won’t affect Firefox but will look
horribly askew in IE.

Of course, there are many sites where style trumps usability,
especially in art and entertainment. If this is the case with your design, it’s
helpful to include a statement on your front page or footer, i.e.
“This site is optimized for Firefox at 1280x800 screen resolution.”
Also, if your site requires Flash or a PDF Viewer, include a link to
download what’s needed.