Originally Posted by Fat Steamroll
Once Serperior gets released into the real world it's going to get a quickban from NU, the tier it currently resides in. Serperior has a unique niche in NU, nothing like what it's Dreamworld counterpart will be using. The realworld Serperior uses subseed or dualscreens in a futile attempt to survive, while also being able to outspeed key mons in NU and prevent them from setting up, laying hazards or statusing it via taunt. Its dreamworld counterpart however is a key pivot in some matches, coming in, placing a substitute and leech seeding its opponent or setting up screens; and spamming leaf storm maniacally only to become stronger and stronger after every use, leaving the opponent in a very tight situation as he/she has to end this monster's rampage before it kills their team. The only thing we'll have to admit now is that this will be the only fully-evolved starter pokemon besides Infernape that is fit to play with the behemoths of the future OU metagame.
I also think it'll deserve a quick ban from NU and possibly RU too, but I think you're exaggerating a little. Leaf Storm only has 8 PP and as the OP mentions, it's still a Grass-type move. It'll hardly be sweeping teams with one move but it will make some giant dents.
And its counters depend mostly on what Hidden Power you run. HP Fire leaves you vulnerable to Lati@s, Heatran, Chandelure, Dragonite...and HP Ice makes you weak to Steel-types in general.