I think all banned pokemon should be banned immediately. But seeing as we aren't smogon, we can have more "complicated" bans. For example, I'd say Blaziken with Blaze should be allowed, Blaziken with Speed Boost should not. So far Smogon have been pretty easy on the bans, everything that has been banned, has been terribly over-powered with no place in a competitive metagame. All this being said, I propose the following bans, Fultix, feel free to add or subtract as you see fit to make it "official" for the site.
Banned Abilities: Moody, Swift Swim + Drizzle is a combo ban, meaning both can not be on the same team, but are fine seperately (Though I think Drizzle Politoed should just be banned outright on here, but I'll leave that up to you Ful)
Banned Items: Brightpowder, Lax Incense, Focus Band, Quick Claw, Soul Dew (currently unreleased, if you have it, you hacked)
Banned Moves: OHKO moves (Fissure, Horn Drill, etc), moves that do nothing but increase invasion(Double Team, Minimize, etc)
Now the important stuff, Banned Pokemon:
Mewtwo
Ho-oh
Lugia
Groudon
Kyogre
Rayquaza
Deoxys-N and A (Speed and Defense formes are allowed)
Palkia
Dialga
Giratina (All formes)
Shaymin-S
Manaphy
Darkrai
Arceus
Zekrom
Reshiram
Speed Boost Blaziken (again I suggest Blaze be allowed, any one using Speed Boost will be DQed)
Also just to add, Latios and the unbanned Deoxys formes are currently on notice, and could be banned fairly easily next round, so just keep that in mind.