The issue is less about levelling up and more about having to clear 2 expansions worth of quests before you can start playing in with the bulk of other players. That's the thing turning people off and it is turning people off. That's bad. Player churn is a constant factor in MMOs and if less players are starting then you have a drop in player numbers.
Think of it this way. Your friends convince you to play an MMO with them. You join excited.
The first 30 levels you find yourself enjoying the game and the story.
Next ten levels your settling in and making steady progress. Your talking to your friend but apart from them rushing you through dungoens you have to complete you don't really get to play with them. One invites you to PotD but while you enjoy it and it gives you EXP you find yourself still having to go back and start grinding through quests again. Still you making progress.
Final ten levels you have the build up and reach the finale! Victory. Your happy with the story and feel you have reached a milestone. Lvl 50 complete! But you still cant play with your friends. Now you start on your crawl through the 2.X content. You might not mind it but still its kind of getting old. You want to get on with it so you can finally start playing with real people. Currently your pretty much playing a really long single player game. However you persevere. You get to 2.55. You watch an amazing cutscene. Finally your into HW!
It takes 2 more levels to realise your still 10 levels before this single player content is over and you can finally join in with what the others are doing. You enjoy the world and you enjoy the story but its all you have been doing for almost 2 straight months and your getting really tired of it. You hit by the time you hit lvl 53 your motivation for logging in has dropped right off cause you know you have several hundred quests still to go and its become a chore.
I see that play out a lot helping new players. Now sometimes they push on and finish. Sometimes they take a break but eventually come back and finish. Often they just log off and never come back. You would be surprised at the number of sprouts I have seen quit in early HW. The MSQ is great and most really enjoy it, the story and the characters but the whole thing taken in one go is generally more than most people want. Its just way too long. In 4.0 it gets even longer. That is the problem.
Now maybe there is a better solution than a jump potion but its not people being lazy. This is a problem and SE will have to find a permanent solution cause its not a problem that will just go away by streamlining a few quests or bumping up EXP earned. As the game gets older this problem will just get more acute.