In my opinion, the changes to healing and mana costs make the game more challenging in general, but not impossible. As a Disc Priest all throughout WotLK (Didn't get a chance to do much in BC, I got to 70 2 weeks before WotLK came out) I got accustomed to having infinite mana and spamming Flash heal and bubbles everywhere. Greater Heal and Heal didn't exist on my bars, and I didn't really worry about Spirit a whole lot. So judging by these facts alone, you could say I'm one of those wrath babies.
So what am I saying about these changes?
I think these changes are the best thing to happen to this game thus far. It was a bit of a shock when, soon after the patch hit that night, I could barely keep my tank and dps alive through Blackrock Caverns and Throne of the Tides. At first I was infuriated like so many others, thinking that Blizzard had broken my class. But as I continued running instances, I realized that my healing style needed some changes. I learned to adapt. Holy carp, I just ran out of mana on a trash pull.All I did was cast Flash Heal on the tank! Smite? What's that? This talent allows it to heal now? When did I get a spell called Heal? Holy crap, it doesn't heal for a whole lot, but barely costs mana either. These are the kind of changes that people need to start noticing. Blizzard didn't buff the crap out of some spells you previously never touched for no reason. There's a very good reason that Flash Heal now costs 5k mana.
So now that Blizzard has made healing a job that you need to pay attention to, rather than a chore that you only do because it earns you shiny lootz faster, healers need to understand what has changed about their class and, most important of all:
ADAPT