Some of the most important (SEO) aspects one should look for in a CMS are:

• The ability to specify the exact URLs of your pages. If you want your page to be called bmw-car.html, then you should be able to do so. You want to avoid a CMS that forces dynamic urls upon you.
  
• The ability to change the HTML Title tags of each page to the last character. You don't want the CMS to dictate anything to you, or force some dynamic title upon you.
  
• The ability to change Headers and Alt Tags. If you can't change these basic things, avoid the CMS.