Your Magento store’s overall look (or design) is determined by the theme you choose. Magento default installation uses the default theme that comes prepackaged with the realease you have installed.
Most people want to customize their design a little (or a lot) to make it their own.

The comparison below summirizes the most important creterias that must be taken into consideration to optimize your decision when considering a new Magento design.

Free themes vs. paid-for themes vs. custom-built themes

Custom-built themes

A custom-built Magento theme is the best option for serious businesses looking for an unique design that fits their company’s identity. Developement cost can be very expensive and you need to wait for several months before you see it completed (coding and testing).
Do not take this risk unless you are sure what you are doing.

Free themes

Free themes are good for those starting out on a limited budget. The problem with them is, a lot are not updated regularly, many are abandoned completely, there is no support for most of them and the quality of the coding is not always optimized.

Paid-for Magento Theme

A paid-for Magento theme is a cost-effective way to launch a new Magento store or relook an existing one.
How to select the theme that best fits to my store?

We asked some of our clients about the creteria they have taken into consideration to make their choices, and they asnwered with the following:

• Pick a theme that relates to your Industry (we will leave it here even if we do not share the same opinion, because colors, fonts and images are easy to modify)
• Check the update frequency: how often the developer updates his theme
• It's a good scalable responsive design (obviously)
• Comes with a built-in mega menu feature - ask the developer how much work is required to build the menu. Is it automated, does it take the category structure, categories are hard coded?
• Ready and easy to install/uninstall and activate/deactivate.
• That the theme is Very well documented.
• Uses Cross-browser Coding
• Comes with Free and verified outstanding developer support
• Looks really attractive and appealing and only contains visual elements that have a real function
• That in the design only products display represents the most important aspect > the design of the shop itself does not play a major role
• That the design of each page type is tailored to the purpose of that type of page
• That the design uses as little pictures as possible in the html (actually only the logo)
• Follows Magento best SEO practices.
• Optimized for Fast Loading
• Uses CSS sprite techniques to reduce number of https requests
• That the theme is supplied with Full PSD files and that those PSD files are easy to edit.
• HTML & CSS Well-structured and commented for easy customization.
• That the call-to-action elements in the design greatly stand out from the rest of the design
• Be Aware of the Terms of Service

Our recommendations:

• Check themeforest themes they have a large selection of Premium Magento themes.
• Ask for advices from reputable Magento developers before you buy anything. They can guide you to make the right choice.

References

Choosing a Magento theme
How to Choose a Magento Theme | Creatuity
Free Magento Themes vs Custom Magento Design | Inchoo
SEO and Selecting a Magento Theme