As far as I can tell, thorns now scales with your mainstat, so the number it shows is actually lower than the damage it will do. Also, there are several legendaries/sets that increase thorns damage/effect. For example, the new lvl 70 set Thorns of the Invoker gives a large bonus to thorns damage, and makes your thorns damage hit all enemies within 15 yards. Another weapon, a 1-handed legendary axe called Hack, applies 75-100% of your thorns damage on every attack. Finally, in addition to the Thorns set, the new lvl 70 Demon set's 2-piece bonus adds 6000 thorns damage as fire, and since the Thorns set is only 4 pieces, you can get the Demon's set bonus with any combination of the pants, belt or chestpiece. Finally, the Flawless Royal Topaz, the highest Topaz available, adds another 5000 thorns damage.
If this all works, then Thorns should be viable if you build for it. Thorns damage now works for both ranged and melee attacks, instead of just melee, so any attack that hits you will proc the thorns damage. Mainstat goes up far higher in RoS than it does currently: most legendaries give you about 500 to mainstat, so your mainstat should be between 6000-7000, meaning that thorns damage will get a very large boost. In addition, as I have shown above, if you can stack up a lot of thorns gear, it should be very possible to get well over 20-30k thorns damage, which after damage boosts should hit for a very large amount of damage. Finally, if you are playing a Barbarian, you have a passive that increases your thorns damage by 50%, further increasing your potential damage. Basically, if all of this information is indeed correct, then thorns damage should be viable, so long as you build towards it.