CRZ Could Have Theoretically Fixed

It's worth mentioning that leveling was also made easier because so few people were around to help out.  Through Vanilla, if you had a tough group quest, you probably had people in your zone on the same quest you could group with. WoW accounts for sale By BC, there was no point - nobody was there any more.  You skipped them and maybe came back later to solo them at a higher level, or if you were lucky, you had a much higher-level friend who'd take pity on you and come carry you through it.

This happens in almost every MMO at some point.  Some have built-in ways around it (GW2's scaling). Others do nothing: I had several players tell me it wasn't worth starting Final Fantasy XI a few years ago because I'd get stuck on un-soloable content and would never find anyone to group with.  

WoW dealt with it by getting rid of the hard leveling content, adding a dungeon finder, and tuning the dungeons with a fairly low set of expectations (because if 3/4 of your dungeons end in failure, players just won't do dungeons any more).

CRZ could have theoretically fixed it too, Where to buy wow accounts but I guess they didn't anticipate implementing it when they "fixed" the world for Cata.

Technically we start around level 15 when we can start entering instances.  All those instances while leveling up?  That's where we originally learned to tank, heal and dps within a group.  Now it's nearly impossible, thanks to speed runs, BoA gear, and impatient players who just want to level up faster.  Proving grounds helps solve this learning problem you speak of.  It does not help! When players exit the proving grounds, they will meet the same impatient players, with BoA gear and speed runs. This thing won't solve anything!

Exactly. Change the stupid "go go go" attitude of the players and you can learn your roll.

As it stands now, I'm leveling a Druid healer and Warrior tank, both in the 40's and it is impossible to actually "run a dungeon correctly."

Asshole players pull everything, run ahead, engage bosses, etc., with no consideration to any other player. If you want to take a dungeon seriously, you'll get vote kicked.

The problem isn't game mechanics, it is players. Get rid of the players and the game would be fine.