Rules of Magic

The OUaT narrative has established several rules for the use of magic. These aren't always applied consistently, however.

Love True and Otherwise

True love's kiss will break any curse (1.12). See True Love.

That magic cannot force someone to love is established in 1.10. Much of S1 makes this debatable, in that with everyone's false memories, they are sometimes able to convince themselves that they are in love with a person according to those memories. Similarly, S2 references a curse that can make someone think they are in love with the curse caster.

"Magic can't make someone believe, dearie. It works the other way around." (3.02)
The Price of Magic

"All magic comes with a price (1.04 and others)."

The evidence so far suggests that the areas of magic subject to the price concept are fairy magic, Dark Magic, and curses.

The Dark One

What happens to Prince Thomas (1.04) can be taken as proof that "magic comes with a price" is objectively true in at least some cases. When Thomas volunteers himself to pay the price for trapping Rumple, the universe listens. After Rumple has been taken, he states that until Cinderella's original debt to him is paid, she will never find Thomas; when Emma takes on that debt, Sean and Ashley are reunited in Storybrooke. When Emma becomes the Dark One, the Rumple figure informs her (5.02) that exacting a price for magic performed is not optional, and she cannot pay it herself; Emma’s attempt to avoid imposing one for helping Robin only delays the matter.

Fairy Magic

The price for Pinocchio's transformation into a live boy is that he must remain "selfless, brave, and true" (1.20), and the terms of the geas follow him to the LWM. Jiminy can be transformed into an insect as he desires, but he must then spend his life in servitude (1.05).

Curses

The Dark Curse explicitly sets down the cost of its casting; the heart of the thing you love the most (1.02) is on the list of ingredients. Later events (2.11) suggest that the price goes further than that, and extends to Regina finally losing Henry when everyone returns to the Enchanted Forest. When Regina creates the sleeping curse turnover to use against Emma, Rumple identifies the price for that magic (1.21) as Henry falling victim to it.

In 2.04, Rumple theorizes that the memory loss that falls on anyone who crosses the border is part of his price for creating the Dark Curse, that if he goes to look for Baelfire, he will instead forget him. What's more, "I brought magic to this world to find Bae, and now he's dead. Magic always has a price, and [the destruction of Storybrooke and the death of everyone in it] -- this is it" (Rumple, 2.22).
You have to wonder why people keep on doing the curse thing.

There does not appear to be a direct quid pro quo attendant on psionic magic or the use of magical objects, but a metaphorical price may apply. Regina says to Archie that "Magic is the way I've always gotten everything" (2.05), to which he replies, "It sounds like magic is also the way you've lost everything."

"My magic has a price. The price of being the Savior is, I don't get a day off" (Emma, 3.10).
A related and so far unanswered question is, does magical power inherently corrupt? Baelfire believes magic itself to be the cause of his father's evil. "It makes good people do terrible things" (Henry, 2.17). "You think you are the diner at the feast, tasting the offerings. A little love, a little darkness. What you don't realize is, you are the feast, and the darkness has tasted you" (Rumple, 3.03). When Emma uses Dark Magic, even for ostensibly helpful ends (5.02) she shows signs of physical transition to being the Dark One.

The Blue Fairy on the other hand says that there is good magic and bad magic, and that hers can be trusted because... well, because she says so (1.19).

Conservation of Magic

Canon is a little muddled on this subject. 3.21 established that when a magic-user dies, their magical works are undone, at least supposedly -- Zelena's death was supposed to lift the curse on Emma (except she probably wasn't cursed really). 4.08 altered this a bit with the idea that magic can never be completely destroyed, when Rumple used the remains of the ice urn to temporarily imprison Ingrid.

 4.11 went further with this, apparently completely disregarding 3.21:

• Ingrid's ice wall remained around Storybrooke until Elsa dispelled it.
• Even after the ice wall was dispelled, a curse remained on the town line that would prevent anyone who left from returning.
• Although Marian was thawed, she soon began to refreeze and had to leave town to save herself. (Unless Zelena did that.)

 After it was used to kill Hades, the Olympian Crystal was reduced to ash, except for one small piece. This was later used by Rumple to contain all of Storybrooke's magic in his effort to wake Belle from the sleeping curse. When the magic was returned, the crystal vanished.

Life and Death

Magic cannot return the dead to life. (1.06, 2.05)

Regina and Dr Whale attempt to use magical methods to return their loved ones to life (2.05), with disastrous consequences. Rumplestiltskin was resurrected by Neal, but the price for that was Neal's own life. Pan wanted to return to the living world by trading places with someone who was alive. The Author's pen can return a dead soul to life, but only while the pen itself is in the Underworld. Killian was returned by divine intervention.

The most dubious entry in this catalog is Zelena, whose continued existence depends on some hitherto-unknown property of her Oz pendant to preserve her life force, which was able to come up with a new body for her once returned to the Enchanted Forest.

"Only the Dark One has life eternal." (2.04)

On a similar tack, no mortal magic can grant immortality. Although we meet several otherwise human characters with unnaturally extended lifespans (the Apprentice, Fendrake), the only true immortals are the gods, those who have drunk from the Holy Grail, and the Dark Ones by extension of this power.

Wonderland Appendix

The rules of magic are presented slightly differently and more concisely in OUaTiW. Magic can’t bring back the dead, force anyone to love you, or change the past. Going along with the idea that “anything is possible in Wonderland,” a way is found in that land to render all of these laws null.

Even Jafar’s spell to fundamentally change the nature of reality, however, could not completely override “magic comes with a price.” Jafar, Amara, the genie brothers, and to a lesser extent Anastasia all paid a heavy one during the course of the story. That has some interesting implications for the OUaT series destination.