Since people are starting to write GBU analyses and there are only a handful of serious posts, here's a look at the metagame through my eyes. I've been playing since the English GBU came out and have garnered a respectable rating (1806), if anyone's wondering.
I face Japanese players almost all of the time, so if anyone has information on players from other countries, I'd like to hear it. (No "English players suck Japanese players rock!!!" posts please.)
Rain, Goodstuffs, and Semi-Stall are the most common teams. Rain is the dominant weather. Sand and sun teams are occasionally seen, while hail teams are nonexistent, but rain teams are always around the corner. A typical Rain team contains Politoed, Kingdra, Ferrothorn, Latios, and two other supporting Pokemon. Rain users typically pick Politoed, Ferrothorn, and Kingdra for battle.
Goodstuffs teams are comprised of bulky sweepers, with a frail sweeper or two. A typical Goodstuffs team has Metagross, Conkeldurr, Volcarona, Garchomp, Rotom-W, and Abomasnow. As in VGC10, Abomasnow serves as a check to weather teams, especially rain. The teams typically lead with Metagross or a frail sweeper. A Focus Sasher is usually in every lineup, so look for the most likely Sasher.
A couple of Goodstuffs teams run an unlikely Trick Roomer (such as Espeon) to turn the tables on fast teams. It's an unlikely but common threat.
Semi-Stall teams are fairly common, and run the usual suspects such as Skarmory, Blissey, and so on. Porygon2 is also a common sight.
Overused Pokemon
Ninetales is an attacking lead for Drought teams, and typically carries an Air Balloon or Shuca Berry.
Machamp is occasionally used on Goodstuffs teams as an attacking lead.
Cloyster occasionally pops up on Goodstuffs teams as a Shell Smash sweeper, using a Focus Sash or a White Herb.
Gengar is used as a Sashed lead on Goodstuffs teams. It carries Destiny Bond for a quick 2-for-1, so slower Pokemon should watch out.
Starmie is used as a Life Orb attacker on Goodstuffs and Rain teams.
Gyarados is a bulky ChestoRest sweeper on Goodstuffs teams, and usually carries a Chesto Berry or a Wacan Berry.
Dragonite is a bulky Multiscale Dragon Dancer on Goodstuffs teams.
Politoed is the lynchpin of Rain teams, and runs a bulky set so that it can stick around and replenish the rain if necessary. It's fairly unpredictable, as it can carry Encore or even Perish Song in the last slot.
Scizor isn't grossly overused, but still pops up on Goodstuffs teams every now and then as a Swords Dancer with an Occa Berry.
Skarmory is a staple on Semi-Stall teams, and runs the standard set with Leftovers or a Rocky Helmet.
Kingdra appears on every rain team as a Life Orb sweeper.
Porygon2 is an irritating all-around wall on Semi-Stall teams, and usually runs its standard set with Toxic. SS teams often send this out into battle.
Blissey is the special wall on Semi-Stall teams, and runs the standard special wall set with Toxic. Chansey is occasionally used in its place, but Blissey is more common.
Tyranitar is used on sand and occasionally Semi-Stall teams for sandstorm, but Hippowdon is the more common Sand Stream user.
Surprisingly,
Blaziken is only seen on Goodstuffs teams occasionally. It uses a sweeping set with Protect, and carries an Air Balloon or a Life Orb.
Swampert is sometimes seen on Goodstuffs as a general wall with Leftovers.
Metagross is a common lead for Goodstuffs, and runs Meteor Mash/Hammer Arm or Earthquake/Ice Punch/Bullet Punch with a Shuca Berry or an Air Balloon. Prepare accordingly.
Latios (and occasionally Latias) are used as Life Orb sweepers on Goodstuffs or Rain teams. A lot of Latis carry Thunder over Thunderbolt due to the constant rain. I haven't seen Hidden Power Fire on a Lati yet, so feel free to switch in Scizor and Ferrothorn (watch out for Trick).
Unsurprisingly,
Garchomp is whored on Goodstuffs teams as a SubDancer or Choice Scarfer.
Hippowdon whips up sandstorms on Semi-Stall teams, and runs its standard defensive set with Yawn. It usually carries a Sitrus Berry.
Goodstuffs teams use
Abomasnow as a check to weather teams, typically using a Choice Scarf or Focus Sash.
Weavile sometimes pops up as a CounterSash lead.
Gliscor is an annoying mofo on Goodstuffs and Semi-Stall teams, and runs a standard Poison Heal set. Some Gliscor carry Guillotine, so take care of these ASAP.
Gallade sees a surprising amount of play in Goodstuffs, and typically runs the Swords Dance set with Shadow Sneak.
Rotom-W feels like the most overused Pokemon on the GBU, as it's abused in Rain, Goodstuffs, and Semi-Stall teams. It scouts with Volt Switch or tries to cripple a wall with Trick in the early game. It runs a defensive set with Sitrus Berry, or carries a Choice Specs or Choice Scarf. You'll see Rotom all the time, so get used to it.
Conkeldurr normally resides on Goodstuffs teams, but Rain teams also use it to take out Rain deterrents such as Ferrothorn, Abomasnow, and Blissey. Conkeldurr runs the standard Bulk Up set and carries a Sitrus Berry.
Jellicent is a general wall on Semi-Stall teams. It uses the standard set with either ability.
Ferrothorn resides on Rain teams. It uses the standard set, but seems to run Thunder Wave more often. It carries a Rocky Helmet or a Sitrus Berry .
Thundurus shows up on rain teams as a simple sweeper.
This is all from memory, so any corrections or additions are welcome.
