Leveling up a new character is one of the most boring parts of this game.
I strongly disagree.
Personally, I enjoy the leveling process quite a bit, since there is constantly something new to experience. You get new skill runes to to try out every level, your appearance changes often as higher levels of armor become available and the different classes have lots of interesting dialogue.
Maybe it's just me, but I like a more relaxed leveling pace, experimenting with skill runes as they become available. I also tend to buy a matching set of new armor as soon as my level is high enough to get a new look, just to see what my character will look like
I even enjoy the conversations, learning little tidbits about the philosophies and opinions of each class.
If higher monster density while leveling becomes an option, cool, but I can't be the only one who actually doesn't enjoy powerleveling, right?
This was my experience as well in leveling: it was fun to get new runes to play around with. However, it became bland fast once I realized that most of the runes are useless, space-wasters that are never used by the majority of the community (and can't efficiently be) in inferno mode +. You need to have really top of the line gear to get some builds opened up--and even then most of the runes are still useless. Take the DH for instance: most DH's are clones of each other in inferno. Rapid fire just opened up, which is great. But, from gear to builds they all use the same thing; barbarians are worse, and wizards are horrible, too.
Simply put, leveling isn't fun when you know most runes are worthless later on (and are pretty much worthless at any point in time); leveling isn't fun when you've done the same zone several times before; and, leveling isn't fun when you run into a pack of 5 monsters every 30 seconds. I've timed it, and of course it's random, but it happens regularly at lower difficulty levels outside the nauseating halls of Act 3's Keep.
Mob density was one of the best changes to Diablo 3 so far--granted it should've been there to begin with, especially having D2 as a template--and I'm surprised it takes this much contemplation to add it in for lower difficulties. If it's fun to fight chaotic waves of enemies in inferno, why wouldn't it be a great experience in lower difficulties? If it's a great experience you're wanting to give new players, don't hesitate on giving it to them 14 hours + of game time in when they've done the same, barren act 3 times over.
Still, thanks for the mob density changes in Inferno; I'll take what I can get.