Magus is not an auto-play in the non land-denial versions.
The only decks he hurts bad enough to completely cripple in one shot are UGW Threshold (Not UGR, due to burn), and Landstill. Either of these decks can Force him on turn one. Threshold can Daze him or just ride the Tarmogoyf they have to Victory. Landstill scoffs at him, as Landstill just leaves its mana untapped and floats the appropriate color mana to STP/Edict/Whatever him. Or, if Landstill's got a Deed down, Magus is irrelevant.
On top of that, Dragon Stompy maindecks anywhere from 4-8 boardsweepers that kill Magus of the Moon. Rolling Earthquake is an auto 4-of, and recent versions are packing Pyroclasm as well (Though maindecking the Clasm is a metagame call.)
EDIT: Also, interestingly enough, I've gone back to running Bloodrock Cyclops over Sulfur Elemental in the Quake/Clasm maindeck version. He synergizes much better. Although if you aren't maindecking Clasm, Sulfur's by far better.

