Even without heirlooms or restedness, levelling still feels too fast to me. I want to see the new quests, but I also can't stay away from instances entirely and that's already enough to throw my questing completely out of whack. By the time I finished the Northern Barrens, most of the quests and mobs there were already grey to me, and all because I had done a couple of RFC and DM runs... I guess it's good for people who like to level using only one part of the game or who really like to pick and choose their content, but for me... not so much.
I chose to make my new troll druid feral but didn't feel like tanking, so I queued as dps... and as healer, since I healed low-level instances as an off-spec very successfully in the past. I do have to say though, I'm now not sure anymore whether that's such a good idea, at least not with random groups. It's far from impossible, but I still found it a lot harder than I expected, even though I had collected a full set of int gear for healing.
Mana was the biggest issue - healers all get meditation at level ten now if they choose to go resto or holy, and I reckon that not having that as an off-spec healer was what made the biggest difference. Spell scaling was another problem: in the past I used to compensate for off-spec healing by only queueing for instances for which I was on the higher end of the level range, using the fact that the extra levels had increased my mana pool to my advantage. However, nowadays the cost of my spells increases with each level as well, meaning that being a few levels higher doesn't help at all - my mana pool will still only be good for x number of spells. If anything I found it more difficult to heal once I hit twenty or so, since it was around that time that I seemed to lose the massive regeneration bonus that all the low levels get these days.
Some of it might also be a class-specific problem however. For example I had a run in which I was a kitty and our healer was a shaman in pretty crappy gear, but his mana bar hardly ever seemed to move at all. After observing him for a little while I noticed that he was making a point of using his super mana-efficient heal, healing wave, most of the time. Paladins and priests get their version at low levels as well, but druids still don't get nourish until level seventy-eight. Ouch! This left me with only rejuvenation, regrowth and healing touch in my toolbox. (Compare the 26-35% of base mana cost to nourish's 10%!) Rejuvenation worked reasonably well, but I needed a direct heal to cover damage spikes as well, and neither regrowth nor healing touch worked really well for that. The former is fast and heals for a good amount but too expensive to use all the time, and healing touch is so fricking big that it's hard to use it effectively as you'll overheal a lot of the time, and that while it's still quite expensive. (In my runs it healed for a little less than 400, with the tanks having between 400 and 600 hitpoints. I could hardly wait with casting until they were near-dead every time!)
Still, I don't want to make it sound as if healing was completely horrible or anything, but the instances as a whole are definitely not as facerollable as before. It's hard to make a judgement on how much more difficult they've become, because it still depends a lot on your group. If you've got a tank in full heirlooms you'll probably be fine. However, I've also had non-heirloomed tanks that were at the lowest level for the dungeon and went splat so quickly that I could barely even get a healing touch off in the time in which they went from full to nil. I actually started pre-casting and cancelling my heals in some cases, the humanity! Talk about something that gave me vanilla flashbacks. Actually, that's probably the best way to describe the state of these revamped low-level instances: they have a certain vanilla WoW feel about them. They'll still go a bit faster, but overly crazy pulls are definitely not recommended. Crowd control is not strictly necessary but can be helpful.
I already mentioned in my last post that Ragefire Chasm hasn't changed much. Mobs have been thinned out and moved around a bit (for example Oggleflint is right up ahead now instead of up in that cave up on the side), they have more health and hit harder now, but otherwise it's more or less the same. I expect that this is the way Blizzard handled most of the old five-mans.
Both the achievement system and the dungeon finder also still claim that the instance has been completed once you kill Targaman the Hungerer, which is just as silly as it was before, seeing how he's only the second boss. However, due to the new dungeon maps which show more skulls further ahead and the fact that there's now a quest at the start to kill Jergosh the Invoker and Bazzalan as well, you should have better luck with convincing your party to do a full clear than you might have had in the past.
The quests at the start are noteworthy as well I suppose. Blizzard did mention that all dungeon quests would be available inside the instance from now on, and they weren't kidding: a full contingent of quest NPCs awaits you, offering about three to four quests for the instance at once. As if the dungeon finder wasn't efficient enough already... still, I'm not complaining. Having everyone on the same quests has its advantages (see above) and it saves you all the running around you used to have to do to pick up all the dungeon quests. Just remember to go back to the instance entrance at the end to hand in instead of leaving the group and teleporting out right away.
My only complaint with this system so far is the rather awkward way in which it works with one quest in the Deadmines. One quest giver there has a whole chain to kill all the bosses in the dungeon, except that you don't know that right away. So you get the quest to kill the first boss, clear the instance - then at the end all the quest NPCs conveniently show up so you don't have to run back, and when you hand in, this quest giver tells you to go and kill another boss. But, but... I just killed everyone in here, you're standing right on top of their corpses, lady! I ended up having to do three runs in a row or so just to complete this one quest chain, which felt a tad silly. Why can't she show up after I killed the boss she wanted me to kill and give me the follow-up then?
The Deadmines as a whole have been changed a lot, presumably so that the level eighty-five heroic version would be on par with other heroics. This means that the boss fights are fairly involved for such a low-level instance, though still not too difficult. Almost all the bosses have some sort of ability that requires you to move away, but you shouldn't die immediately if you don't - you'll just make your healer cry. Still, compared to the usual tank-and-spank in other dungeons of that level that's a huge difference.
Also, all the bosses (with one exception) are different from the classic Deadmines. If you're Horde, you'll probably just go "What's going on? Who are these people?" and then kill them anyway. If you're Alliance you can level an alt through Westfall (which I did afterwards) and learn all about the story behind the changes there, with little nods to pretty much all of the new characters. If you're at all into lore, it might be worth levelling an Alliance alt just for that, even if you're usually Horde.