Specifically, the biggest problem is armor. Bear Tanks found their niche in BC through massive amounts of AC, as gear with lots of extra armor allowed them to weather massive physical blows handily. However, since most non-tier gear will now be shared with Rogues, we won't be seeing that extra armor, since no Rogue would want gear that wasted too many points on extra AC. Thus, If this itemization trend stays, bear armor will drop drastically.
Similarly, since bears don't get the same AP and agility bonuses that cats do, nor the same strength bonuses (to block and parry) that other tank classes get, there's some concern that they'll fall behind in threat generation and mitigation in other areas as well. Quite simply, Bears either need their own unique itemization, or they need talents or baseline abilities that make up for the weaknesses of Rogue gear for bear tanking.
If there is one bright spot, it is that Ghostcrawler has specifically said that they want bears to viable end-game tanks and cats to be viable end-game DPS, and that they aren't quite there yet. So at this point, it might be best to wait and see what knobs they turn (turning knobs seems to be the new term for class balancing) to put bears where they need to be.
On the more optimistic side, we can drink potions (now that you can only drink one potion per battle) and use Windfury (now that Windfury Totem's mechanic has been completely changed) in feral forms now, so that's something.