I agree...I levelled a disc/holy priest to 70, a resto shammy to 70 and a resto druid to 70. The heals are very different and how you play them are very different as well. Although I enjoyed my resto druid, I have respeccd my druid to balance/resto so that I can be a dps/utility/offhealer because when a run doesn't need a healer I still wanted to be able to go.
What I learned about druids is that they don't "spam" heal like a holy priest or shammy can. When you "spam" heal you end up drawing so much aggro you'll never get rid of it. They are indeed a preemptive healer who will quickly run out of mana (and get waay to much aggro) if you try to play them like a priest. Their biggest strength is the fact that they can keep steady heals over time but can still do instant heals if needed. I find them to be the most versatile healer as they have heals over time..a front-end heal paired with a heal over time...an instant heal..and a heal over time with a heal at the end...also the ability to make the target's HoT's an instant heal (swiftmend?) if needed. The biggest weakness that I could see is that they aren't very good group healers except when they have to save everyone with tranquility (which rocks btw..just is overpowered and has far to long of a cooldown to use for regular group heals) When you get a druid and priest together who can trust each others heals it makes an incredible pairing!! (I think that in instances such as Gruul's and SSC each of the healers bring something to the table and there should be room for priests, resto druids, resto shammies and pallies as they all have strengths that compensate for the weaknesses of the other classes)