Thalia, Heretic Cathar is one of the most exciting cards from Eldritch Moon for Modern and this is one of the best potential homes for her — I would cut one of the three copies of Mayor of Avabruck and move the Meddling Mage to the sideboard to make room for two copies. In conjunction with Reflector Mage, Collected Company, and Thalia, Guardian of Thraben, it's not hard to see how good Thalia, Heretic Cathar would be in this deck. The mana-denial aspect of Thalia, Heretic Cathar doesn't have the same support from this deck as it does from Hatebears (Leonin Arbiter, Ghost Quarter, etc), but instead it gets the Human tribal support. I'm not sure which one will prove better, but certainly both decks welcome Thalia, Heretic Cathar with open arms!
The deck casts a mana producer or Champion of the Parish on the first turn and then follows it up with a lord (Mayor of Avabruck, Thalia's Lieutenant) or a Mantis Rider. Have you ever attacked for four with an exalted Mantis Rider on the second turn? Let me tell you, it feels almost as sweet as attacking for five on the second turn with a double exalted Champion of the Parish! Both of these things happened in the videos:
The backbone of the deck is Collected Company, which can be flashed back by Jace, Vryn's Prodigy. The deck relies on Path to Exile and Reflector Mage for removal. The deck only needs a small amount of disruption since it presents such a fast clock. Thalia, Guardian of Thraben and Meddling Mage represent the only main deck ways to disrupt non-creature spells, though of course Thalia, Heretic Cathar will soon change that.
In the sideboard we get more disruption spells that are tailored toward answering specific problems. The deck can afford to have this kind of sideboard because the maindeck is highly synergistic rather than just a collection of individually powerful cards. This means there's only a few cards to sideboard out each round and can therefore play more specific answers in the sideboard instead of having more broadly applicable cards that can be boarded in for a variety of matchups.
Against creature-light decks, cut Path to Exile and/or Reflector Mage. Against heavy removal, cut Mayor of Avabruck. Against spell-light decks, cut Thalia, Guardian of Thraben. Meddling Mage can also often come out.
Stubborn Denial is the most versatile sideboard card. it comes in against almost anyone, though it's mostly for spell-dependent combo decks.
Mark of Asylum is an answer to Pyroclasm, Anger of the Gods, or any strategy that aims to burn creatures.
Rest in Peace comes in against any graveyard-based combo deck such as Living End, Thopter Sword, Melira Company, Goryo's Vengeance, Open the Vaults, etc. Bringing in one copy against Tarmogoyf decks is also fine.
Stony Silence is primarily for Affinity, though it also comes in against Tron, Thopter Sword, Open the Vaults, and any deck reliant on artifacts. I also bring in one copy against decks running AEther Vial.
Meddling Mage comes in against strategies reliant on resolving a specific card such as Ad Nauseam, Living End, Ensnaring Bridge, or (against us) Supreme Verdict. I'll sometimes name the creature bounced with Reflector Mage too.
Spellskite comes in against decks reliant on targeting a specific creature (Infect, Bogles) and can also come in against decks that are especially crippled by Meddling Mage; The Spellskite can protect Meddling Mage from Abrupt Decay or other point removal spells.
Lantern Scout comes in against aggro decks and burn decks where games often are decided by racing.
Finally, Gideon, Ally of Zendikar is great against midrange decks that aim to outlast us by killing all our creatures with one-for-one removal spells. Gideon also answers Night of Souls' Betrayal by making an emblem to effectively Cancel it out.
This deck is super fun to play and quite powerful. And it only gets better with Eldritch Moon. I can't wait to take it for another test run in a couple weeks when Thalia, Heretic Cathar is released!