The mobility and CC of Druids is just hard to overlook. Personally I don't understand how people can recommend anything over it for BGs.
If you have game sense, you will know when to go to bear and you will live the rogue opener, nature's grasp, fairie fire and travel form out. HoTs are incredibly useful and totally shift momentum in larger team fights. You got your instant BIG heal with NS to keep warrior alive in critical moments. Stealth on demand is priceless, letting you decide which fight you want to take, when you want to take it. With engineering you've got tools for nearly every situation. The only thing druid lacks is magic dispel and silence, however once TBC hits and you acquire Cyclone you're so far ahead of other healing classes there's no decision to be made in my opinion.
You mentioned Shaman, which is also a decent BG healer and has good utility tools to help your team. Shamans however are huge targets in BGs, you're almost never able to freecast, have 0 ability to escape snares, but you at least can wear mail + shield. In my experience shammy needs a babysitter to be able to babysit the melee battering ram. Windfury totem does turn some losses into victories however.
Both classes have moderately high learning curves, you're playing the opponent's class as much as you're playing your own, if that makes sense. For me, it's druid.