Instances are the best place to level, get items, and do supporting quests. Here is a list
of instances you should definitely check out if you have a group or find yourself grouping
with other players often.

The game is tailored so you cannot just do instances, unless of course you take some
level-lag in between. The optimal WoW group is a group of people who always play
together and just do instances and kill X of Y quests. If you could achieve such a group,
you will level quite quickly.