Yahoo has dozens of features that CBS does not have. Surely you already know that since you claim to have played both. If you love CBS so much what are you doing here?
Yahoo has the ability to write notes on your team page, CBS does not.
Yahoo has far superior stats, splits and situational stats compared to CBS where you can hardly see anything and when you can it is incomplete.
Yahoo has several types of graphs on the standings page, CBS does not.
Yahoo has far more functionality on the trade proposals process for comparing players and stats.
Yahoo doesn't charge you $120 for an inferior product.
Yahoo's live stats tracker during games has far more data and options than CBS's does.
Yahoo's Disabled List prevents team owners from permanently stashing healthy players on the DL, CBS does not.
Yahoo provides more scoring system options than CBS does.
Your team page on Yahoo tells you much more detail about your players' upcoming opponents than the CBS team page does.
Yahoo has entire sections of good info that CBS doesn't have, like the recent add/drop trends, which players show up most often on the best fantasy teams, player raters, constant player updates that are much more accurate than the ones on CBS, which players are "hot" etc etc etc. The list goes on forever.
Get into a Yahoo league and surf around all the tabs and menus to see all the dozens of options that CBS doesn't offer. Yahoo has infinitely more data and features available than CBS or ESPN. It is like night and day. Not even close to being similar. Yahoo is by far the best fantasy baseball provider -- and that is why they dominate the market.