Wobbuffet is also not broken. First of all, due to the prominence of the Deoxys-D HO teams, most effective teams have at least one Pokemon that will be able to KO a Wobbuffet that has taken prior damage, so it is generally only getting a single kill at most, if even that. It's not a Pokemon that requires excessive preparation. Any player with a team weak to Wobbuffet is likely not only weak to a single Pokemon, but also weak to one of the most established team archetypes in the metagame.
That being said, because of team preview, a competent player should understand that if his/her Landorus kills something with a Choiced Earthquake, it is liable to being revenge killed the subsequent turn by any opposing Wobbuffet. In the same way should a competent player understand that if his/her Landorus kills something with a Choiced Earthquake, it is liable to letting any opposing Dragonite set up freely on it. If Dragonite would sweep the player's team, then using Earthquake to revenge kill something on a team with Dragonite a bad idea; if Scarf Landorus is necessary for a team to win, then using Earthquake to revenge kill something on a team with Wobbuffet is also a bad idea. I would argue that these are all easily understood consequences and do not make Wobbuffet broken. This (the nature of team preview) is a difference between Gen 4 and Gen 5's Wobbuffet that you neglected to mention, by the way.
In regards to defensive Pokemon, teams that carry defensive Pokemon must allow for the possibility of free set-up turns anyway, Wobbuffet or not. Lucario doesn't need Chansey to be locked into Toxic to set up on it, nor does Volcarona need Forretress to be locked into Rapid Spin. These are once again consequences of using those Pokemon, ones that can be abused easily without the need of Wobbuffet support.
That being said, because of team preview, a competent player should understand that if his/her Landorus kills something with a Choiced Earthquake, it is liable to being revenge killed the subsequent turn by any opposing Wobbuffet. In the same way should a competent player understand that if his/her Landorus kills something with a Choiced Earthquake, it is liable to letting any opposing Dragonite set up freely on it. If Dragonite would sweep the player's team, then using Earthquake to revenge kill something on a team with Dragonite a bad idea; if Scarf Landorus is necessary for a team to win, then using Earthquake to revenge kill something on a team with Wobbuffet is also a bad idea. I would argue that these are all easily understood consequences and do not make Wobbuffet broken. This (the nature of team preview) is a difference between Gen 4 and Gen 5's Wobbuffet that you neglected to mention, by the way.
In regards to defensive Pokemon, teams that carry defensive Pokemon must allow for the possibility of free set-up turns anyway, Wobbuffet or not. Lucario doesn't need Chansey to be locked into Toxic to set up on it, nor does Volcarona need Forretress to be locked into Rapid Spin. These are once again consequences of using those Pokemon, ones that can be abused easily without the need of Wobbuffet support.





















