Every class now has three real trees from Classic, The Burning Crusade and Wrath of the Lich King, seven tiers deep. Each talent is a real talent of that class; where its original meaning does not exist here (rage, mana, threat) it does the closest thing you can feel, and its tooltip says so.
You can train up to 51 points. A talent in tier n needs 5 x n points already spent in its tree, so the bottom-row capstone costs the 31st point: Bladestorm, Divine Storm, Chimera Shot, Killing Spree, Penance, Thunderstorm, Living Bomb, Chaos Bolt, Eclipse, Dancing Rune Weapon and twenty more. Only one capstone fits in a build, so a deep spec still chooses.
Talents never add buttons: they change which spell leads your kit, add a spell to it, quicken cooldowns, and add procs on crits, kills, hits taken and escapes. Support trees carry the party heals and party buffs.
One-time refund: the trees were rebuilt, so every point you had trained is refunded in sigils at exactly what it cost. The lobby tells you once; retrain in the Armory.
New prices: a point costs 1 + P/5 sigils (rounded down, P = points already trained): 1, 1, 1, 2, 2, ... The first 20 points cost 58 sigils, all 51 cost 306.

The Death Knight is offered to everyone, for every race: create one from the character screen, add one as a companion bot, or pick it in a party.
Plate-wearing melee: Blood Strike leads the kit, F is Death Grip (pull an enemy 3-30 yards away to you), and Runeblade Parry is the innate. Frost, Unholy and Blood bring Icy Touch, Obliterate, Howling Blast, Plague Strike, Death and Decay, Unholy Blight, Death Strike, Blood Boil, Dancing Rune Weapon and Army of the Dead, with their Wrath visuals.
Three trees from Wrath: Blood (tank: leech and kill heals), Frost (crits and slows) and Unholy (diseases and ghouls), with Dancing Rune Weapon, Howling Blast and Summon Gargoyle capstones.
Gear: plate and two-handed or one-handed axes, maces, swords and polearms drop for the Death Knight across every zone; Death Knight-only pieces (Scourgeborne and later) drop only for it, and Shadowmourne is its legendary.
Ten Death Knight companions join the bot roster, from Darion Mograine and Thassarian to the one who Death Grips the wrong caster, each with their own lines.

A companion no longer plays without talents. It follows its persona's favourite tree to the capstone (most companions pick a damage tree, a few tank or heal), then a second tree.
It gets as many points as the party's players have on average (a guest counts zero), so a fresh party gets fresh companions and a veteran party gets veteran ones.