At level 40 you can keep two talent builds and swap between them, according to Blizzard's class deep dives. You still respec at a class trainer in a major city. The second build is a chance to cover a role you can't play while leveling.
Our picks for the second spec
Warrior: Arms for questing, Protection for dungeons.
Paladin: Retribution for questing, Protection or Holy for groups.
Druid: Feral for questing, Restoration for dungeons. Or keep Feral and split between a cat and a bear build.
Priest: Shadow for questing, Holy or Discipline for dungeons.
Shaman: Enhancement for questing, Restoration for dungeons.
Hunter: Beast Mastery for questing, Marksmanship for groups where a pet is a liability.
Mage: Frost for safe questing, Fire or Arcane for group damage.
Warlock: Affliction for questing, Destruction for groups.
Rogue: Combat for questing, Subtlety on PvP realms.
Why it matters
Tanks and healers find groups faster. A damage dealer with a healing spec can skip the queue and level through dungeons. Full talent builds for each spec come from the live client week. Until then, each spec page says what's new in that tree.