Rotom W is absolutely deadly in this metagame. It's incredibly bulky even without any investment in defenses. It takes about 50% from CB max attack Roobushin (and can OHKO back with Specs Hydro Pump) and takes about 35% from Doryuzu's Rock Slide and about 45% from Doryuzu's Return (and once again can OHKO back with Hydro Pump). It has a great typing, especially for a defensive pokemon.
It's got decent speed as well. Although 86 may not seem like much, it is enough to outspeed certain pokemon such as Suicune, Kingdra, Heracross, and Nidoking. If given a Choice Scarf, this decent Speed Stat becomes great. It can outrun Adamant Tyranitar after two Dragon Dances, Gyrados after one, Manaphy, and Starmie. These pokemon are extremely threatening pokemon, but luckily most can be OHKO'd and outsped. If you are going for the Choice Scarf set, Rotom H isn't a bad idea either. It can OHKO Scizor, Jarooda, and Nattorei. You also might want HP Water on a Choice Scarf set to nail Shandera.
Finally, this thing packs a punch, especially with Choice Specs. Not many things can take even resisted attacks from a Specs Rotom. Its STAB has a very good STAB combination, but it becomes even better with HP Ice. Special Attacks are becoming more important with bulk up abusers like Roobushin and Zuruzukin running around. Rotom has the power to take them all out with its super powerful STAB moves, Thunderbolt and Hydro Pump. It can nail walls like Blissey with Trick, rendering them useless.
Overall, Rotom misses its Ghost typing, but it is still just as powerful as it was in Gen IV.
Rotom-W@ Choice Specs
Modest
4 HP/252 SpAtk/252 Spe
--Thunderbolt
--Hydro Pump
--HP Ice
--Trick
Rotom-W/Rotom-H@ Choice Scarf
Timid
4 HP/252 SpAtk/252 Spe
--Thunderbolt
--Hydro Pump/Overheat
--HP Ice/HP Water
--Trick/Volt Change