I went from 70-73 as resto on my first druid since I had finished TBC as a healer. This was the point where I just couldn't stand the lack of speed anymore. The first few levels in Northrend aren't too bad because of the way the starting areas and quests are laid out, especially with a good number of short chains leading straight into Nexus and UK.
But as the XP demands increase, so do the dungeon quests decrease, and in dps-spec soloable group quests start taking over.
I started another druid a while back, and I have been consequently leveling as resto. I've reached level 30 now, and I don'f feel handicapped yet. Random dungeons are plain awesome to level, and they result in gear that make questing fast enough for my taste. I hardly get to do quests, though, because the demand for healers seems high enough to keep me in dungeons pretty much as long as I want.
I mean after all Azeroth has been very well cleaned up anyway. Most of the grind has been removed thanks to increased drop rates for quest items. You're getting the most of your points from actually completing the objectives rather than killing things along the way, and being able to just prowl to whatever needs killing is rather effective.
I'm also looking at how druids start. The first 10 levels you're playing as caster anyway, whether you want to or not. No talent points to worry about either yet. Bear form is so damn slow that for the most part you stay caster until you absolutely have to shift to survive (or get an Iron Counterweight if you can). Early cat form isn't exactly a winner yet either.
On the resto side, though, Nature's Focus is an instant winner, and at 22 you have Intensity maxed out for plenty of mana for the very much uninterrupted Wrath spam. Azeroth gear has always been high on Spirit, so you can be sure to get the most out of it.
Balance needs to wait until 37 for full Dreamstate, and cats don't get the only decent solo-finisher Ferocious Bite until 32.
My plan is to get dual-spec right at 40 when Moonkin form becomes available. There are still plenty of interesting Dungeons left at that time, like the newly split-up Maraudon, or Zul'Farrak, but eventually you WILL end up in BRD for a good amount of time, and I'm sure as hell not having that. Felwood and Un'Goro are a so much better place to spend the early to mid-50s. A little bit of Dire Maul or Scholomance then before heading to Outland... (I think they became available at 56 when I recently finished Azeroth with my pally)
In short, a dps spec is always better whenever questing becomes a priority. Anything else can be done as resto without it being too painful.
One thing to keep in mind, though, is that you can just as well heal with a Balance spec. Normal dungeons are forgiving enough, and the only one spell that sets me apart from Balance druids at this level is Nature's Swiftness. The majority of the talents that actually increase healing are so far down in the tree that it's damn hard to even feel a difference. Also, many of the Balance talents are on the bottom line quite similar to what you get on the resto side.
For example, Intensity is good and great and all, but Moonglow is just as good (Cheaper spells vs more regeneration) and there are a couple more talents like that.