Gliscor isn't used much anymore, since the metagame is so fast-paced and offense. It's been replaced by Landorus-T, who also gets the benefit of Intimidate to sponge physical attacks. However, even those aren't very common.
Yeah, Dragons usage is dying down a lot as well. The only two you really see a lot anymore are Latios and Latias, particularly the latter, which is used as a Life Orb Tank set to eat up the all-too-common Rain-boosted Scarf Keldeo Hydro Pumps. Stuff like Dragonite and Salamence aren't too common anymore, because DDnite is easily revenged by any common Scarfer, and ScarfMence is outsped and OHKO'd by Scarf Keldeo's Hidden Power Ice. I think you're right, the drop in Dragons usage is indicative of the fast pace of the metagame.