Secondly, I'd like defined "harder" in this context. I think leveling a healer or a tank (with that spec) is tougher and more time consuming than leveling a DPS class. Yes, tanks could level as a DPS spec but then would suffer the consequences in their grouping ability and/or be forced to pay the 1,000g to allow for dual speccing. DPS classes don't face this obstacole.
I disagree with this. First, you could always tank and heal normal instances regardless of what spec you have. The base class always includes the necessary tools to heal and tank. Back in 2008, I wrote a post about Retribution Tanking for WotLK normal dungeons. It's really only when you hit end-game heroics and raids that you need to spec for your role.
Second, the new Dungeon Finder makes leveling as a tank or healer significantly easier than it used to be. I've taken a warrior from 15 to 32, and it's been blazingly fast. I would rank it as faster than questing. Not to mention that character has much superior gear for her level. She's almost all blues, without really trying.
With the Dungeon Finder, it is quite possible that tank and healer leveling has become easier than leveling as DPS.
Also, I think there's a fair argument to suggest that both tanking and healing are more stressful roles than DPSing because you are really carrying the weight of your group or raid on your shoulders. I know people will vehemently disagree with me on that point but so be it.
In my opinion as a long-time healer, the idea that "you are really carrying the weight of your group or raid on your shoulders" is an illusion. It is an illusion born of unfamiliarity with the role and the fact that you are an obvious point of failure.
It's true that if the tank or healer makes a major mistake, the group wipes. If you're new to healing, this idea can seem intimidating. This is especially true when you're faced with a new role. DPS is familiar, it's what you did all through your leveling experience.
So when faced with an unfamiliar role, where failure is obvious, it's not really unexpected that the roles of tanking and healing would be invested with more weight than they deserve.
However, once you tank or heal for a while, the unfamiliarity goes away. Once you get as comfortable with healing as you were with killing mobs, it doesn't seem harder or easier, just different.
The first time I tanked on my warrior at 15, I was very nervous. Now, at 30, I'm jaded. I know my role, I know my purpose, and I can take care of my end. That's how fast the acclimation process goes.
You don't have the weight of the raid on your shoulders. It just seems like you do at first. And practice cures you of that illusion.