This belongs in new/returning player forum. *Moved*
Just a list of the decks you might see (roughly ordered from most popular to least popular)
Reshiram/Typhlosion Prime
Zekrom/Pachirisu/Shaymin/Tornadus
Magnezone Prime/Yanmega Prime
Gothitelle/Reuniclus
Less Popular Decks:
Vileplume/Reuniclus/Suicune and Entei LEGEND/Donphan Prime/Zekrom/Pichu(/Tropical Beach)
Magnezone Prime/Emboar/Reshiram/ Rayquaza and Deoxys LEGEND
Donphan Prime/Yanmega Prime/Zoroark
Donphan Prime/Reshiram/Zekrom
Reshiram/Emboar 19/Emboar 20/Rayquaza and Deoxys LEGEND
Yanmega Prime/Weavile (sometimes with Donphan Prime)
Blastoise/Floatzel
Vileplume/Yanmega Prime/Sunflora/Mew Prime/Aipom/Jirachi
Mew with fast cards like Cinccino, Zoroark, Tornadus, and Yanmega
The format is basically a triangle between the faster decks (Reshiram, Zekrom, Donphan, Yanmega) which beat the slower decks (Yanmega/Magnezone), which beat the manipulation decks (Reuniclus with Trainer lock, Magnezone/Emboar), which beat the fast decks.
Blastoise/Floatzel randomly falls in there somewhere because it can beat Donphan and Reshiram through weakness and Reuniclus by sniping it, but it can't do very much against anything else.
Mew is there because it can beat Gothitelle, and with fast attackers it has a chance against a lot of other stuff.
Nothing is really broken in this format. Pretty much every deck (and almost every card) has a hard counter. It creates a very diverse format with a lot of decks, but often winning comes down to getting paired against good matchups. Staples are Pokemon Collector, Pokemon Communication, Junk Arm, and Rare Candy (in Stage 2's.) Most decks are also running 8-11 draw supporters (Professor Juniper, Sage's Training, Judge, Professor Oak's New Theory, Copycat.) The first three are my favorites. Pokegear 3.0 is also staple, imo. PlusPower has gotten a lot of play.