1) Theme Width and Columns

Typically, WordPress themes come in 2-column or 3-column formats, with widths ranging from 500 pixels to 960 pixels wide. If you're blogging for non-profit purposes, a 2-column theme can look more compact and reader-friendly. Since you have less images of products or links to other sites to display, you can focus exclusively on the content without leading readers away from your site.

On the other hand, if you're blogging for profit, you may want to consider a 3-column WordPress theme that will be able to accommodate your Google Adsense, Chitika and Text Link Ads codes comfortably without squeezing everything in the content area. 3-column themes allow room for expansion, but in the event that you've filled up all available space with ads, then it's time you removed the non-performers and use only the advertising services that work for that particular blog.

2) Use of Images and Icons

A theme with images and icons can look good, but it rarely increases your web traffic or subscriber base. In fact, most "A-list" bloggers have plain vanilla themes with a simple logo on top. Reducing the amount of images also means faster loading time and less stress on your servers. This vital aspect of server load become apparent only if you have tens of thousands of visitors a day, but it's worth designing for the future.

A image-laden theme also distracts readers from the content itself. This is the reason why blogs like Engadget and Tech Crunch use images intensively in the content areas to add value to a post, but the theme itself is simple and rather minimalist.

Ideally, a theme should allow you to use your own header image for stronger branding purposes, yet replace images and icons with links and text, or just not use them at all unless absolutely necessary.

3) Compatibility with Plugins

Another time-sucking activity is installing plugins that improve the functionality of your site. There's a plugin out there for almost everything you want to do with your blog, but while most of them are free and easily obtainable, it's not always easy to install the plugins and insert the codes into your WordPress theme.

If your theme is too complicated, it may be a headache to even insert that one line of code you need to make a plugin work. This is often the case with advanced AJAX-based WordPress themes that have too many files and heavy coding. I've always preferred a simpler themes that stick to the default WordPress theme as much as possible, so I can cut back on the learning curve and just get on with my life.

Remember that the purpose of your blog is to deliver timely, relevant content to your readers, Any theme that preserves or improves the reader experience is good, any theme that subtracts from the experience is bad.

4) Search Engine Optimization

A lot can be said about search engine optimization, but at the end of the day if you have content worth reading eventually you'll get the rankings you deserve. However, that doesn't mean that you don't need SEO; it merely means that as far as optimization is concerned all you really need to do is to make sure:

(a) Your tags are formatted properly, with the name of the post first followed by the name of the blog - some themes can do this automatically without modification to the code or use of a plugin

(b) All your blog content titles use the H1 tag, with the main keywords used instead of non-descriptive text for better SEO relevance

(b) Your theme has clean source codes, and if possible all formatting is linked to an external CSS file which you can edit independently

5) Plug-And-Play Ease of Use