Leading the dungeon party's direction and to be the object

This doesn't mean someone who is DPS geared and spec'd can't act as a tank in an instance. They can do anything but may find it difficult to find quality DPS and healers who want to party with them on an ongoing basis unless they're over-geared for the instance and giving run-throughs.
Leading the dungeon party's direction and to be the object of the enemy mob's hate, taking damage and protecting the rest of the damage dealers and healers in the party is the basic role of a good tank.
It's largely expected that for a group to be successful, the most important parts are that the tank and healer are solid. Having poor DPS can slow down a dungeon run in a severe way as well but having a poor tank or poor healer almost always spells doom for a party.
Knowing that we're all in a process of learning about how to play our classes as we're leveling through to the level cap, so excellent healing or tanking is not required. What is required is that the tank can judge the quality of the group's healer, so it knows how hard and fast the party can be pushed without having to spend time getting ready to go again after a wipe.
Setting the pace is one of the toughest things for a new tank to learn, especially since Wow has been going for many years and it's likely you'll get at least one member of your PUG party who is high on knowledge about the particular instance, but low on patience for waiting for you to start the fight.
A good tank will know that Buy RS Gold is using a passive-aggressive, non-verbal way to say let's go faster. It will also practice setting up the marks and starting the battle as quickly as possible. For a new tank, marking is a huge learning curve.