So my son has been actively out there leveling his toons using the new Dungeon Finder feature. For those of you that have been playing it almost exclusively with your level 80's, it may come as a surprise to discover that it does actually work just fine with both Classic and Burning Crusade level dunegons. The tool automatically figures out which dungeons are appropriate for your level toon and even color codes them similar to quests to designate which ones will be easy, medium or hard.
In any case, my son has had fantastic luck leveling some of his lower level toons almost strictly using the new dungeon finder tool. He hasn't had particularly decent groups - typically only getting one or two bosses down in any given instance before the group falls apart, but it's still enough to generate a very impressive amount of xp for him.
As more people pick up on this as a way to level their alts, I think this could have a very positive impact on the level of end game play. The reason being that it is now much more possible to level your toon using the spec and talent build that you'd like to use for end-game.
For example, if you want to play a holy priest, most people would have leveled as Shadow, using a talent build and rotation really focused on up time and survivability. They then would have made the switch to holy once they hit level 80 - and worked on running heroics essentially with an almost brand new type of character that they know very little about playing.
Now, using the new Dungeon Finder tool, you can really level using the character that you want to play - whether that's a holy priest, a holy paladin, a resto druid, or elemental shaman, which will make you much more skilled with that player once you finally do get to 80. Also, leveling primarily through the Dungeon Finder, you should also end up geared better than you would strictly by questing, which will also make the transition and gearing up process for end game easier.
So, for those of you with low level alts, I'd encourage you to get out there and try the new Dungeon Finder. Respec to what you really want to be when you grow up and go out and have fun!