Well its no secret that a nicely designed blogs tend to have repeated visitors.Problem is If you are not good at PHP and CSS and don’t have the money to spend on a custom blog design, then your only option is to go for a free theme that looks nice.Well looking nice isn’t the only thing to consider when getting a free theme , below are few things that are often overlooked but equally important when choosing a free theme.
1. Load Time
According to me one of the most neglected things when choosing a theme , Most people go for nice looking themes with large images where it takes a long time to reload , just imaging one of your articles getting stumbled or dug and your site getting a decent traffic. Well if your site takes a long time to load chances are readers will skip it fast , losing potential readers 
.Even if its theme with few images if the PHP is written badly it might be slow to load.So After changing your theme its better to check and compare the load time with the previous theme. http://www.iwebtool.com/speed_test is a good place to determine your blogs load time , be sure to check about 3 times and get the average time.
2. Customizable header.
To stand out from the rest you need a cool looking header , although you don’t know any designing tools you do know how to upload a file right ? There are Wordpress themes which provide you this ability.Then all you have to do is find a nice image to impress your visitors.Try to find something relevant to what you blog about.
3. Social Bookmarking enabled
Its true there are many plug-in’s where you can add social bookmarking , but most of them require you to add some code in some files , which you cant do if your hosting or Wordpress , so its always good to find a theme with social book marking as a part of the theme. Like this theme 
4. RSS Feed in the top and clearly visible
Well this blog is not the best example of it. But since this theme had many of the other attributes I chose this one. I’m already working on a nice looking button to replace the one at the moment. I cant remember the number of blogs I have visited once and moved on because they don’t have a RSS subscriber clearly visible.
5. Number of Comments clearly visible
People are often afraid to take the first step , if your post has one or two comments the chances of the new visitor commenting greatly increases. Of course the reader have to see the number of comments first.So choose a theme where the comments are clearly visible.
The first one is kind of alright although the size of the font could have being improved.The second one is great and the third is the worst of the lot , not only its small but it’s light gray in a white background .
6. Ad Support
Well this is something you cant do in blogs hosted in Wordpress because java script is not allowed , but if you are hosting on your own server its useful to search for a one with 125 x 125 ad support and ad sense support.There are plenty of themes where you just have to enter your ad sense publisher id and everything rest is take care of. But remember if your fairly new to blogging and not getting much traffic it is better to avoid displaying ads for now. If you do install a theme with ad button visible don’t leave them blank , fill them with affiliate buttons .
I just stopped by your blog and thought I would say hello. I like your site design. Looking forward to reading more down the road.
Robert Michel