Brad has asked me another great question that I thought all of you out there would want to know, so I'm putting it down as a new post. "Are you finding the mobs your killing/grinding to be of equal difficulty for your level?"
The leveling so far has been quite easy. Obviously this depends on your class, I think level 60 is the hardest transition if you aren't already uber (epics, etc). My tank and rogue both had 5-man blues or lesser and both experienced more down time fighting mobs as a result. My hunter and druid were both able to handle the mobs fairly easily. Hunters, just because they are hunters :) The druid I respecced 31/21 feral, and she had a reasonable selection of 5-man blue gear for tanking already so she's been able to do almost as well as my hunter (sometimes better if she can sneak to the boss, lol).
So I'd say if you are still in Tier 1 or lesser gear, you may have to work a little harder than the next guy. But the upside of this is that you will appreciate the quest rewards so much more than the next guy. I believe it was Tobold's blog I was reading a few days ago where he talks about the same issue with quest rewards. Most of them are Tier .5 to Tier 1 quality stuff and if you don't have that level of gear yet, you'll be totally amped about all the new stuff you're getting. I know I was! Kaliope has a whole new feral set with 7k health, 12k armor and can dish out 145 dps in bear form. Before I got to BC I was only in the low 90's on dps with bear. The only piece I'm still using from pre-BC is my Mark of Tyranny, although I haven't ditched my Unyielding Maul in case I need to tank for a dungeon run.
Healer gear is kind of a different story, I've gotten 5-6 upgrades including a crafted leather set that had a couple of nicer items than my old stuff (gloves and boots, never had stellar gear on those slots). Most of the quest items in the last two levels (63-64) have been excruciatingly close to what I have. In fact I got a kilt that is just a hair under my Animist Leggings, but it's prettier so I kept it ;) I'm guessing by 65 I'm going to start upgrading my DM/ZGish healer stuff. But it's all been of minimal benefit to me since I've been leveling feral anyway. So if your current neato gearset isn't one you can use to level (non-shadow priests, I'm looking at you) then it won't do you much good that you have it.
With that in mind, I actually prefer the undergeared route to the overgeared one. Who wants to plow through 6-10 levels of quests and never get a new item? Boring if you ask me. I look forward to completing a quest when I know there's a nifty new weapon coming my way. And by the way, I'd like to toss in here that there are some awesome feral staves in the new game. They've added a lot of new categorizations which include heal bonuses, spell bonuses and more. For example "of the Invoker" gear has stats plus spell bonus. "of the Elder" is usually stats plus a heal bonus. I bought a level 62 "bear" staff off the auction with +44 Stamina, +29 Strength and a whopping +119 attack power in feral forms. This isn't a special quest item, just a regular old loot drop.
Anyway, back to the original question. I have yet to find any quest that is just impossible to solo, except for the elite ones. Some are a bit more challenging than others (high mob respawn, low quest item drop rate) but all can be done. If your toon was a challenge to level up in the first place, it will likely be the same now. I don't think it's any harder and due to the high volume of quests and high quest XP (10-12k per quest) leveling is actually easier in some ways. Also, once you complete the first round of quests in Honor Hold, you'll be tougher anyway. So any shortfall starts to balance out after your first few hours of questing.
The chatter I've heard from other players seems to match my own take on this, I'm hearing from other classes that they've respecced away from raid mode and back to leveling mode. Most of the mobs have 4k-5k hit points, closer to 5k once you get to 61-62 mobs. If you are one of these classes that has trouble soloing, stock up on potions to help you get through the first few hours until you can upgrade your gear with quest items. Also, make sure to save plenty of runecloth, you'll need to get your First Aid to 330 to learn the new bandage recipes. This will probably take 6-8 stacks of runecloth. I can't tell you how many ppl I've heard whining in general chat for donations of runecloth - sheesh!
In general, I don't think the mobs are harder. I do think if you are an undergeared player you should probably stock up on buff foods, heal pots, etc. Just enough to get through the first few hours of game play until you can swap in new armor and weapons. As I said in one of my earlier posts, each class will end up with 2 new weapons (or equivalent slot item) and at least half a dozen new pieces of armor by the time they get halfway through level 60. I believe the intent was to help gear up those who aren't walking around in Tier 1 or better, so those players can handle the new content as well as the uber folks. And they'll have more fun questing because they can actually use the rewards they're getting. So don't worry if you're not epic'd out - Blizz has got you covered ;)