Three classes can tank: 
Warrior, 
Paladin and 
Druid. Shaman can hold a small group for a while as Enhancement, but Blizzard doesn't present it as a tank spec.
Warrior: the safe choice for raids
Protection Warriors get Thunder Clap in Defensive Stance and Rage from hits that ignores armor. Shield Wall no longer shares a cooldown with Recklessness and Retaliation. If you want to be the raid's main tank from day one, this is the default.
Paladin: tanks from level 10
Seal of Fury arrives at level 10. It adds Holy damage per swing, gives an absorb shield and makes Judgement taunt from 10 yards. Add baseline Consecration at 20 and a 
Protection Paladin can tank dungeons from very early. It's the biggest change to any tank in Forever.
Druid: the flexible one
Feral Druids tank as a bear and deal damage as a cat in the same tree. Potions and enchants now work in form, and Lacerate is a talent at 16 points. If you want to tank some nights and deal damage on others without respeccing, choose a Druid.
Our pick
For a first character, Paladin: it's tough while leveling and can tank dungeons early. For raiding at 60, Warrior is still the safest bet. We'll revisit once there's real data from the live game week.