So if you have the $75 annual fee Blue Cash preferred, you need to spend $2500 or more in groceries per year to make it worth it over the 3% cashback on groceries (but annual fee free) Blue Cash Everyday.
A lot of people easily did this by buying not only their groceries at grocery stores, but got 6% cashback on a lot of other items too - by buying gift cards (Shell/Mobile/BP, Amazon/Best Buy, Gap/American Eagle, etc) at the grocery store (single retailer gift cards typically have no fee to buy beyond money loaded on them).
This was always officially excluded per cashback policy, but Amex could never tell what you were buying because all they got was the total purchase amount and merchant category code - they could tell you spent $252.31 at a grocery store, not that $102.31 was groceries and you bought $150 in gift cards.
Unfortunately, as of January 15th, 2013, you will only earn 6% cashback on the first $6,000 in grocery spending per calendar year. This should be printed on your October statement.
The card should still be worthwhile if you break $2,500 in grocery spend as you'll outdo the rewards of the no annual fee card (including deducting the annual fee from those rewards), but for the highly dedicated people buying a lot of gift cards who would break $6K, this may be a big deal.
More importantly, realize credit card rewards are never set in stone. Sometimes the change is positive (e.g. Discover upped the 5% quarterlies to $1500 in eligible spending from a lower amount), but great rewards can always be capped (e.g. Sallie Mae recently capped the grocery and gas 5% cashback on one of their Visa Signature cards). Always watch your statements.