Tempo mage using secret synergies.
This deck moves incredibly fast in the early game with cards like Kirin Tor Mage, Medivh's Valet and Arcanologist. Mana Wyrm has high damage potential and can set your opponent within burn range very quickly.
Counterspell is fantastic at protecting your board, and can be a win condition against Druid or Big Priest. A previous weak link in this deck was Mirror Entity, but Explosive Runes is a perfect fit for the deck and replaces it well.
Cheating out Kabal Crystal Runner for 0 and 2 mana is also extremely satisfying and hard for your opponent to deal with.
Hard Mulligan for Babbling Book and Mana Wyrm. Two-drops are also fine to keep, especially if you are going second. Never keep secrets in hand. The reason secrets are good in this deck is because they can be cheated out and provide synergy. Individually, they are pretty slow cards on 3 mana. Even when you have both Kirin Tor Mage and a secret, it is generally best to mulligan both unless you also have a turn one play.
Note: Mana Wyrm is one of the more valuable cards in the deck because of the damage it can snowball. Sometimes it is a fantastic turn one play, but if you have nothing to follow it up consider holding it. For example, turn one Mana Wyrm into the opponent's Flame Imp is pretty poor without a spell to follow it up.