Pre-BC you can heal everything just fine (excluding BRD and DM). Once you hit OL the bosses hit much harder...perform set abilities, etc. and the pulls require more CC and coordination. I am currently a 66 resto druid with 685+healing and nearly 8k mana. Don't let anyone convince you that feral healing OL instances will be easy. In fact, I wouldn't take anyone other than a dedicated healer. This isn't because you won't be capable with a solid healing set. It's because DPS tend to be scrubs, CC is sub-par, and finding a non prot-pally tank that can hold even two mobs at once seems to be nearly impossible on my server.
You will be fine as a main tank healer. However, when the AoE damage starts flying. (Shirrak in Sethekk, Pandemonius in Mana Tombs, hell even Nazan can be nasty to melee dps.) There are so many fights that require the DPS to be all over their job. My best example would have to be the Omor fight in Ramparts. He slaps on the nastiest debuff, Treacherous Aura that causes X amount of dmg every 3 seconds to everyone near the debuffed player. I tried this fight with 2 warriors and 2 rogues. Needless to say, when your DPS is bunched it's hell. If you are in a good guild or know some smart people, then go for it.
But I wouldn't count on being able to compensate for the inherent idiocy of other players. As a feral healer, you simple lose all your "Oh ****" buttons, no more swiftmend or natures swiftness as well as inherently weaker heals.
(P.S. If you plan on doing it don't take Nurturing Instinct, the thought makes me /cringe)