In What Makes a Design Good?, I talked about the technical aspects of a good design, like site and user goals, branding, and distinction.
I did not however answer the question of what makes a design look good. What makes one blog visually attractive, but makes another one ugly?
There is no mathematical solution, but there is an artsy one. A good looking design is a combination of a number of factors. If your blog succeeds in each of these areas, then the overall effect will be an appealing design.

designtext 

Image by Nir Tober.

Structure

I have a friend who won’t go into a certain sports store because the layout is too confusing. Men’s clothes next to the women’s, kids next to adults, football tops out on their own etc. I’m sure there is an order to it somewhere, but I can’t see it.
There is a lot of information on a web page. You can order your page in your own mind, but if the ordering is not blatantly obvious to a first-time visitor, they’re going to be overloaded and leave.
Structure and grid-based design are the corner-stones of a well laid out blog.
Further Reading: Designing With Grid Based Approach

Balance

A good design is easy to take in. It allows your eye to glide around the page naturally, without being continually drawn to a certain area of the page.
Think of a see-saw. If you put too much weight on one-side, it will tilt in that direction. But if the weights are equal, it will balance.
In the same way, the weights of the different areas of your page should be equal. The weight of a specific element is affected by its size, but also by its color, its position, and even the detail within it.
Further Reading: Principles of Design: Balance

Color

color 

Image by Vygotskij.
Color is a scary topic for many people, but it must be braved. The most sure-fire way to ruin a great design is to choose an awful color scheme.
If you don’t trust your eye to pick a good color scheme, there are many tools out there to help you.
Further Reading: Basic Color Theory

Small Details

A person can appreciate complexity in a design. The small details make the overall effect.
You don’t need great works of art in your header, or fancy features in your sidebar. The small details in your site will communicate the level of effort put into a design all by themselves.
It’s working on those details that takes a design from good to great.
Further Reading: Design Is In The Details

Clarity And Emphasis

No reader cares to read all of the content on your page. Most want to read as little as possible. They only need the gist of your points, and then they move on.
Clarity in your design allows one element to differentiate itself from its neighbor, and emphasis on your most important elements, such as post headlines, indicates clearly where the reader should look.
Ample whitespace is a favorite tactic for achieving this. It lets your design breathe, preventing readers from being overloaded and confused.
Further Reading: Give My Web Space

Remarkability

remarkable 

Image by Orgutcayli.
This is Seth Godin’s word. You normally hear about it in the context of products and services, but it applies to web design as well.
There are two routes that a design can take. Safe and not safe. You can choose a simple color scheme, a regular 2-column layout and a normal Arial font, and make a decent blog. It won’t be anything spectacular, but it will do the job.
A great design does something more. It goes further than the regular web site and it makes itself unique. Great designs have a certain aspect that makes them better than other sites. Something about them that is worth remarking upon.
Further Reading: 25 Incredibly Artistic Websites

Functionality

Design on the web is not based solely on appearance anymore. Even on blogs, the functionality on your page plays a large role. What do you think when you see beautifully laid out threaded comments? Or well implemented Gravatars?
A good blog is much more than a diary with comments. It allows for you to interact with the content, and makes your experience much more enjoyable.
Further Reading: 35 New WordPress Plugins for an Effective Blogging Experience
When you see a web page with great functionality, obsessive attention to detail and no flaws to be found, it gives you high expectations for the site. It says a lot about the blogger.
There are far more than 7 elements to a great design. These are merely my favorites. What do you think makes a design great?