If you've played WoW for five years you know that while the game has held onto a lot of very important thematic elements the difference in game play between now and the original release is staggering. Even just over the last year since the release of Wrath of the Lich King the game has undergone some really significant revisions. Of course revisions are always made with the endgame in mind, but Blizzard has not skimped on those revisions that are targeted towards the leveling and questing experience.
When I leveled my first character, a druid, the best way I could find to beat enemies was Moonfire and auto-attack with my staff. Cat form, all abilities included, seemed to do less damage than just swinging whatever stick you had. Attack spells were costly compared to my mana regen and didn't really hit that hard (it was almost impossible to get any spell power gear). Moonfire and swing beat an enemy in about 20-30 seconds, and that was the best I could do.
It's a real testament to how bad the leveling process was for druids that when I later switched to a warrior I leveled as protection and was impressed by how easy it was. And these were the days when Battle Shout cost 20 and parries and dodges generated no rage (*way* before devastate).
Blizzard at some point decided that you should be able to run around and beat monsters, complete quests and go up levels pretty much no matter what class you pick and no matter what talent tree you go up. There are still a few trees that are obviously sub-par, but not as sub-par as the leveling options that many classes used to have.
But despite the efforts that have been made, a lot of classes need a serious overhaul for leveling, and it has a lot to do with talented spells and new abilities. By "new" I don't necessarily mean all that new, I just mean abilities that have been introduced since Classic. Talented abilities and these new abilities are generally not available at low level, and that means that abilities that you are supposed to have to make your character do things are abilities you might not get until level 40, 50, 60, 70 or 75.
Go make a paladin. So you cast a spell that last for 30 minutes then you auto-attack. Okay, no big deal, its just level one. Go train your level one ability, it's an aura you put on when you auto-attack. Well, still level one. What does level four hold? Judgement, a spell you actually cast, and you can cast it once every 10 seconds! So generally you cast it when you engage and enemy then you auto-attack until the enemy is dead. You also get blessing of might which makes you do more damage when you auto-attack. Level ten? Put up your buffs and auto-attack, though you can stun someone every minute if you like, but it doesn't do any damage. All through to level 20 you get a better selection of auras and blessings to have on yourself when you auto-attack. At level 20 you finally get another spell you can actually cast to defeat your enemies. It has an 8 second cooldown. It also costs so much mana that given the fact you have no mana regen, you will have to drink every three monsters if you actually try to use it, and it probably doesn't do a lot more damage then you normally do anyway. Probably best to auto-attack. At level 40 you can finally put a point into a talent that gives you mana back when you judge, so you have mana to use a spell other than judgement, though it's really level 42 before you can max it out and use it effectively. At level 45 you've put two points into another talent that will let you use Exorcism to hit your enemies, although only on an on-crit proc with your bad crit chance, and only ever 15 seconds even if you are lucky. By this point you've got a +15% bonus and a +10% bonus to the damage of Crusader Strike, an ability you don't even have yet. At level 50 you finally get Crusader Strike, and you are finally actually playing a retribution paladin. You hit buttons when you fight your enemies instead of just auto-attacking.
Blizzard wasn't happy that all elemental shamans did was cast lightning bolt so they gave them another spell to mix things up a bit. It's called lava burst and you get it at level 75. What about shamans who want to level up as elemental? They can cast lightning bolt until they are 75. Arcane mages basically aren't even playing until they get Arcane Blast at level 64.
Strangely enough, for hunters they actually got it right. They give hunters their mana regen aspect at level 20, despite the fact that it was added to the game in the expansion. They also lowered the cost of shots so hunters can afford to do more than auto-attack until they get steady shot at level 50.
There are actually easy solutions to all of the problems I've given here. In the case of mages, shamans and various other classes that get now-pivotal abilities late in life, just make more ranks of the ability and give it out sooner. Sure, you introduced in it an expansion, but that doesn't mean it has to be forever above the pre-expansion max level. For paladins the fix I often talk about with a paladin wielding friend of mine is to give paladins crusader strike at level 20, and to make it cost nothing. Is that somehow going to cause problems? Are holy paladins going to be broken because they can swing their terrible damage to spell power trade-off fast one handers for 200 by wasting a GCD? Having the kay abilities of a tree embodied by it's talents is fine, but when you hit level 10 and get your first talent point, you shouldn't look at a tree and say, "Well, half of this does nothing until I put the 51st point in." And no one should be auto-attacking their way to level 40, even if they get nice buffs to do it with.