Leveling through BC era content makes me what to delete WoW. That is all.
Is it the speed of it for you, the content in general, or something more specific?
Well, for me, it is like this...
1). HORRIBLY itemized quest rewards. You have to go through a huge portion of HFP to get a 2H Strength weapon (or use a 2H Agility one), for example. Or how about all of the Str/Int or Str/Agi plate? A lot of items you get in Outland have the wrong stats, useless stats, etc. And there's a whole slew of weapons with nothing but +Attack Power or +Spell Power. No Strength, no Int, no Agility, nothing.
2). Piecemeal quest rewards. So you can do a whole zone and you might fill 4-5 pieces on your paperdoll. Eh, okay. Go to the next zone, you do half that zone, you might fill a piece or two more. By the time you reach 68, odds are you still haven't gotten a (insert piece here) and are STILL wearing that piece from Blasted Lands.
3). Horribly outdated questing model. Seriously, who enjoys the "Collect 12 things from mobs that have a 10% drop rate" quest, like the Timber Worg Tails, or the 15 Mushroom Samples, Bog Lurker Tentacles, Bog Lord/Parched Hydra Samples, Eel Filets, etc? These quests take up HUGE amounts of time, are parts of large quest chains, and reward next to nothing in most cases.
4). Outland has existed for years, which means I've done it many times. Doing it yet AGAIN just makes me go bleck, even if #1 to #3 weren't an issue. It just gets old after awhile, and I'd like to see something New.
5). Herbalism and Mining sucks. Herbs are too few, and Ore... well there are plenty of complaint threads about Fel Iron being too stinking hard to get. It isn't as hard as many people say it is, but it is still a bit too rare IMO. Skinning blows even worse -- you get scraps like every other place, but unlike Northrend, and Cata+, you get scraps from EVERYTHING you skin unless it is Lv69+. And not only do you get scraps, you also get Crystal-Infused Leather (which is 99% useless now), and also Clefthoof Hide if you go to Nagrand. Fel Scales can actually be useful, but only for a couple recipes while leveling Leatherworking and it doesn't even take that many IIRC.
At the very least, at least they seemed to have either toned down the ridiculous mitigation mobs in HFP have, or, the new class changes allow you to overpower it easily. I remember the sheer hatred of the "Whoosh" and "Tink" sounds because it seemed like mobs had a 30% dodge/parry chance or somecrap.
Now, compare those to Northrend...
Yes, Northrend does take a little longer, BUT...
#1: Quests reward better gear with fewer pieces having bad itemization. Yes, there's still the whole Stamina Overload going on in Fjord/Tundra, but even with that, most pieces are still good (except for Boots for STR class, you'll be wearing those HFP gaudy gold-colored boots until at least Dragonblight or Grizzly Hills). I will admit, though, there are far too many pieces of jewelry missing Primary stats (all secondaries, nnnngh).
#2: Every zone in Northrend fills every slot in your paperdoll, though I remember some trinkets and decent weapons being a bit deep into some quest lines in some places. It is nowhere near as bad as Outland, though.
#3: Most quests are better designed, with fewer aggravating quests (though I really wish they'd get rid of the Overcharged Capacitor requirement in Borean Tundra. Why is it that Alliance has a quest that requires a crafted item, but Horde doesn't? But at least that quest isn't that important and can easily be skipped without losing too much. Much fewer "bad drop rate" quests (I think some of them like Grizzly Hill's bear flank quest) have been buffed since Wrath as well.
#4: Northrend hasn't been in the game as long as Outland has, but yes even Northrend does get old sometimes.
#5: Herbs are MUCH easier to find, while Ore... Saronite is easy, Cobalt... just takes a little patience (and/or someone to run you through UK for a guaranteed 2-3 nodes). Herbing experience is much better than in Outland, too. You can get 78->80 on nothing but Herbalism if you wished quite easily. As for Skinning, At least in Northrend, there are only 4 different materials you get from skinning, and three of them are rare as they require specific mobs: Jormungar Scales, Icy Dragonscales and Nerubian Chitin. There are many, many many mobs in Northrend to skin if you do not want Scales or Chitin. So you are guaranteed to get Borean Leather as long as you don't skin a Dragonkin, Jormungar, or Nerubian. Also, you are guaranteed to get at least a full leather if you skin anything that's Level 75+ IIRC. Anything in Sholazar Basin will give you full leather and no scraps.
So, those are my opinions of why Northrend > Outland and why I don't mind Northrend as much as I do Outland.