Thorns is capable of being quite a powerful skill in Normal Difficulty. The damage returned gets insane at higher skill levels, and it is a perfectly viable strategy to use to complete the entire game.
It does however lose its use as the player progresses into Nightmare and Hell. The higher difficulties give massive boosts to monster hp, whereas their damage doesn't increase at the same rate; thus it takes longer for monsters to die, meaning they have to hit the character more. This in turn is a problem, for while monster damage doesn't increase proportionally to their hp, the damage increase is noticeable for the player, and since Thorns requires the monster to deal damage in order for it to be returned, this tactic becomes quite deadly for the Paladin in the long run.
Lore[]
An eye for an eye is sometimes not enough. Those who would strike the emissaries of the Light had best take warning, for retribution shall be swift and certain. The might of your blows shall be felt a hundred fold unto you! [1]
References[]
↑Diablo II Manual, page 81. Blizzard Entertainment, 2000.