The exception to the rule are Wizards, and the Channel Divinity/various Word powers.
Wizards: They have a spellbook. When they get a Daily attack power from their wizard class (levels 1, 5, 9, etc), or when they get a Utility power (2, 6, 10, etc) they pick 2, and put those powers in their Spellbook. Then, after every Extended Rest, they pick which power they would like to use for that level. Thus:
A 1st level Wizard has Acid Arrow and Flaming Sphere in his book. WHen he takes an Extended Rest, he decides today he will have Flaming Sphere. He cannot cast Acid Arrow until he takes an Extended Rest and swaps it out.
Also, you can't swap dailies that don't match that level. A 5th level wizard could not put Flaming Sphere in his 5th level Daily slot; he can only put fireball or Bigby's Icy Grasp, etc.
Channel Divinity: Clerics and Paladins receive a Channel Divinity class feature. You can only use one of the Channel Divinity powers per encounter, even though you are given two (so, a cleric can either use Turn Undead or Divine Fortune in an Encounter, not both).
Healing powers: Healing/Inspiring Word are encounter powers, but they can be used twice in an encounter. However, they can only be used once per round. So you can Heal this turn, and heal the next turn, and then the power is used up. (This is similar, but different, from the Paladin's Lay on Hands ability; the LoH can be used every round, up until the Paladin runs out of healing, and then he's out for the rest of the day.)