Stuff like Slowking, Tangela, Scolipede and Miltank shouldn't be going under the radar, especially when Slowking can tank most of the tier and is almost impossible to OHKO.
I've seen loads of Scolipedes and Militanks.
Tangela lacks offensive presense. For me, it just turns into Screen Setup Fodder, and my Altaria couldn't care less about being seeded if it gets some free DD's. It has Roost, after all.
Slowking is a special defender, and NU is primarily physical. Slowking might not get OHKO'ed by a lot [By the way, things like Pinsir, and Scolipede can probobly OHKO Slowking, as well as boosting attackers], but he's gonna be taking more than 50% from non-resisted attacks from powerhouses like Rhydon, or things like Rapidash's Megahorn/Wild Charge.
95/80 physical defenses is bulky, I'll give Slowking that, but it's still not much in the face of some of the attacking forces in NU. Slowking simply cannot tank things like Guts Swellow as well as it should. Although, to be honest, there's not a lot in NU that CAN tank things like Gust Swellow. A defensive evolite Onix was taking over 30% damage from Facade...
That said, I'm one of those players who constantly underestimates defense. I'm almost completely incapable of playing Stall myself, after all...
EDIT:
Just played a Slowking. It took over 25% from Rapidash's Flare Blitz, and 60% from Rhydon's EQ.
On the other hand, Wish Shelgon is a complete peice of work.