While search engines certainly drive content to websites, the next big driver is social media sites. The ability for a CMS to push content out to a Twitter, Facebook, and LinkedIn, either automatically as the content goes live, or at certain times of the day is a definite plus point to have in a CMS.
While pushing content out to social media is a good way to get people talking about your content, it's also useful to have the ability to let readers make comments and to discuss the content on your own site.
The advantage to your site is the free user-generated content will add to the perception that your site is constantly being updated, additionally it also helps in SEO. However there are drawbacks. Unless you vet the content as it arrives, then you won't have any control over the comments and you could become home to unwanted spam comments – most CMS systems have plug-ins to filter out spam but they're not fool proof or will filter out content that contains URLs.
CMS for mobile devices
CMS software products are designed to build websites for desktops, however there are an increasing number of users who want to access content on mobiles, tablets, IPTV etc. You should make sure that at the very least your CMS can scale-down content to fit on a mobile screen and cope with touch screens.
Mobile should be a key consideration. If the CMS can't adapt to mobile then it should at the very least be able to output content to an RSS feed – the de facto conversion standard for mobile website software - so that the content can be adapted into a mobile-friendly format.
Although many CMS systems include analytics they're never going to be as sophisticated as dedicated service such as Adobe's Omniture or Google Analytics. Your CMS should be able to accommodate analytic tags and be able to pass dynamic information, such as the page name through to the analytics tags if required.
This should also work on the mobile sites as well as the desktop sites, otherwise you will only see part of your traffic, and you won't be able to see how your users work between the different channels.
Finally, as we said earlier before you specify a CMS look at how the content is currently produced for your site. Talk to everyone from the content creators through to the content doorkeepers, and also talk to SEO, social and analytics specialists before you go ahead and make a purchase.