first, a disclaimer - i have never given the a CBS league more than a passing glance. that said, the following features are so head-and-shoulders above what i've seen on any other fantasy league management site, that I'd be amazed if CBS (or any other site, for that matter) offered more than one or two of the following features that mfl offers:
1. league "skins" - which are essentially pre-packaged appearance options. with one mouse click, you can change all league pages from looking like the default mfl.com skin to a halloween-themed skin to a draft beer skin to a huddle-themed skin - note those are just 4 of 30 mfl.com-provided skins. cutomers who know industry-standard CSS can create their own skins, and have literally unlimited control over all aspects of their league page appearance.
2. 60 different league "home page modules" that can be displayed literally anywhere on your league home page - two column layout, three column layout, unlimited "rows" in the league home page layout, etc. - in other words, if your league survivor pool is really important to you, you can put those results on the top of your league home page. if you're not running a pool, don't use this home page module.
3. all 60 home page modules can be "embedded" in any other web page on any site that you'd like - for example, you can include your league standings report directly into your personalized google home page, or any other web page that you'd like.
4. RSS feed of key league data.
5. if you happen to be a developer yourself, a developer's program whereby key league data is available in XML format, allowing you to extend the product as you see fit.
6. mfl gameday, a PC-based program with a ton of live scoring display options, and also audio integration so that your PC can announce touchdowns, teams in the red zone, etc. to you.
7. no advertising on league pages. no spam email because you signed up for a league.
8. trading of future year draft picks.
9. literally unlimited waiver periods per week. you want your worst-to-first waivers to run multiple times per day? no problem.
10. an active on-line community, where you can offer your ideas for site improvements (and get direct responses from the site owners!), and share other customization ideas with other league commissioners.
this list is not meant to be comprehensive - just an example of many, many things that mfl.com offers that (i'd be willing to bet!) CBS (or Fanball, RTSports, ESPN, Yahoo, FoxSports, etc.) most likely does not offer.
edit to add:
11. ability to access the site.