I get that question a lot, everyday. So I will answer it here once and for all.
The answer is a clear no, going to Northrend at level 68 is a not a good idea, you are not doing yourself a favor. There are many reasons why it is so:
- Level requirements on quests in Borean Tundra and Howling Fjord. There are many quests with level 69+, 70+ and even 71+ already in Borean Tundra and Howling Fjord. If you go there at 68, many of them won't be up for you and you'll have to go back to those zones later, which is a waste of time. That means more running around to the same spots where you could have done everything in one run.
- Fighting mobs higher than you is not efficient. You'll have to fight level 71 mobs at level 68. Sure that's not a big deal, but fighting orange mobs isn't very efficient. They die slower and do more damage to you.
- Level requirements on quests in Dragonblight and further zones. There are already level 72+ and even a level 74+ requirement quest in Dragonblight. If you start Northrend at level 68, there's no way you'll hit 74 in Dragonblight, even if you complete both BT and HF. (unless you play with rested xp all the time and do instances I guess)
- You'll miss about 2K gold worth of quest. If you skip half of Outland and go straight to Northrend, here is what will happen once you hit level 80: all those Outland quests you skipped will be grey to you. That means no extra level cap gold if you go back to do them. Because if you're an achiever, you WILL go back to do them, for the reputation, the gold, the lore and the zone achievements, right? So why not do them while they are still giving you their full XP reward? If you do them before 70, while they are still yellow or green to you, you'll get the full exp reward from those quests, and you will ding 80 in Northrend much earlier, with several northrend zones left to do. And that means tons of gold from questing at level 80. (And I will release guides for each zone in Northrend). If you do it my way, you'll probably end up 2k or 3k gold richer once you're done with all the quests.
- Difficult group quests. There are some difficult group quests in Northrend, being two level ahead will make it much easier (you'll be able to solo or duo some of them, which you could never dream of accomplishing if you'd go there 2 levels lower). For example in Dragonblight, there are about 8 group quests which really are difficult. You can't solo them, and duo'ing them is also very difficult depending on your class mix. If you go there with 2 level 74/75, you'll probably pull it off. If you go there with 2 level 71/72, you'll probably fail and have to look for more.
- Gear upgrades don't make enough of a difference. The gear upgrades from Northrend quests are nice, but honestly, there isn't much of a gap between green quest rewards you get from Nagrand, Netherstorm and BEM and what you get in BT/HF. It's not enough of a difference to make it worthwhile going there so early and sacrificing all the above, really. I speak from experience, I have tried both methods.
- I went there at 68. I've tried going there with my level 68 Rogue in quest greens (horde side), and then I went there with my other rogue, level 70, in quest greens (alliance side). The difference was really noticeable. It was much easier and efficient with the level 70, not to mention it allowed me to include every quests in circuits, without having to go back and worry about level requirements.
- Overcrowding. Have I mentioned that Northrend is also quite overcrowded at the moment, which slows you down when questing? Try finishing Outland before Northrend, you'll love it. The outland zones are almost empty, you'll have no competition on the quest objectives and you'll have a great time.
There is a HUGE and WIDELY spread misconception among players:
They think that being higher level than the zone's average quest level is bad. They also think that doing green quests or killing green mobs is bad.
This couldn't be further from the truth. It's exactly the opposite! Green mobs die much faster, do much less damage to you and give about 90% as much XP as a yellow mob.
This is even more true for quests! Green quests give 100% as much XP as yellow quests, but are much easier to complete.
Do yourself a favor and listen to my advice: Do NOT skip zones unless you are so far above that the quests are grey to you. As long as it's green, it's great! Don't skip content!
Listen to me and you will level faster and easier. And once you hit level 80, you will be richer, have more reputation and achievements.
All it takes is a little self-discipline, resist the urge to go to Northrend asap, and in the end you'll reap the rewards.