The difference with Rhyperior and Skymin that I can see is this:
Rhyperior looked good due to its relativly good stats and Solid Rock, plus a fairly decent movepool.
Skymin looks good because its stats pretty much rape (Better speed then most Pokemon), along with a good attacking stat (SpA). Not to mention that Rhyperior never needed something to be sacrificed to be able to do away with it, which I can easily see Skymin requiring, something to come in, soak up the 80% chance of losing half your SpDef, fainting, then bringing out something with Ice Shard, only to have them switch out. Weavile can use Pursuit and Ice Shard but even that would require good predicting. Finally, anything switching into Skymin is at huge risk for losing half its SpDef, I saw something somewhere where someone showed that Blissey is 3HKO'd by Air Slash if it switches in on Seed Flare and loses SpDef. Im not even sure if that was counting the Seed Flare Damage..
Heatran is still one of the biggest threats in the metagame, no where near Rhyperior, or Skymin, on the opposite end of the spectrum