The Wrath of the Lich King (WotLK) Beta now has many of the instances open and available for players to adventure in. Azjol-Nerub is the third level-up instance in the expansion and is located along the north-western edge of the zone of Dragonblight. This instance is a part of what once was the empire of the spider like Nerubians.
As you can see in the video, the instance is insanely fast, being only 3 bosses with a few packs of MOBs as guards before each one. Total run time is usually around 30 minutes.
You can find the preview guide to the instance here: Azjol'Nerub Preview Guide.