Blizzard Quote [source]

Thorns is another affix we feel leaves a lot to be desired -- not just in co-op games, but in solo games too. It's not a bad stat, per se, and there many builds that can benefit from it, but it definitely needs some tuning before it can really be considered competitive. We don't have anything planned for the short term, but for the long term we've considered various improvements and tweaks like allowing thorns to benefit from primary stats, building in additional passive support, as well as having one of the gem bonuses give a boost to that style of damage. (We've also tossed around the idea of adding another affix that's very similar to thorns, but has a much more universal function and would be able to benefit all classes [i.e. ranged + melee damage] without the need for passive skill support.)