The last time I dipped into leveling dungeons in WoW, it was a terrible experience. People in heirloom gear completely overpowered the instance, making it the zergiest of zergs. You didn't even have enough time to hand in quests.
In the leveling revamp of the last patch, heirlooms were reined in. They're still quite good, especially with the experience bonus, but they're comparable to dungeon blues.
Low level dungeons are actually a great deal of fun now. I've even rolled a newbie tank just to do instances. It's not excessively difficult, but the game rewards steady killing of group by group. Sometimes you can handle two groups, but it can be a bit stressful.
I healed a Stratholme run and we wiped a couple times due to accidentally pulling extra packs. The group even spontaneously started using crowd control to make life easier!
If you haven't tried a low-level instance in a while, I strongly recommend giving them a shot. Create an Allied Race character, and you'll start at level 20, making you eligible for instances right away. Queue times are pretty good, too. I'm seeing around 8 minutes for a DPS, 1-2 minutes for a healer, and instantaneous for a tank, of course.