The main goal of the deck is to flash in Notion Thief on Turn 4, and Turn 5 drop a Day's Undoing which results in a 14 card draw for you and an empty hand for the opponent. Sometimes, this even results in a next turn Tasigur, the Golden Fang, since you have to discard 7 cards at the end of your turn (meaning the delve cost is easy to pay).
The combo is somewhat slow for modern (turn 5) and isn't a complete lockdown, though, so the deck's true focus is its cantrip, control shell. It has a lot of gas with Wall of Omens and Gitaxian Probe, so moving through the deck isn't tough (also why only 20 lands) Additionally, it can adapt to a variety of different decks after game one. Sideboarding can result in a stronger control deck instead of a combo focus. To keep your opponent on his/her toes, the token aggro mode with Lingering Souls compliments well with the Timely Reinforcements. Many times, the Timely Reinforcements is a true value card since the land base can likely place you around 15-12 life after round 2 (if you're really playing aggressively)
In the past, I've tried focusing more on the Day's Undoing + Notion Thief
combo as a solid win (more bounce spells, Cyclonic Rift, etc. I've found that by focusing on the combo as your sole win condition, you usually end up durdling more than you'd like and then your thief gets removed and your opponent draws 7 cards (ie not a great thing). Having more early threats and cantrips really helps to throw the scent off on the combo, and has resulted in more wins in my experience.
I'm more of a counterspell control player, but the deck can be modified to lean more to the black side if that is your preference. Replace the Gitaxian Probes with Thoughtseize or Inquisition of Kozilek. You also can throw more creature removal in with Doom Blade, Go for the Throat, or any other removal you want.
EDIT: After testing, the heavy control shell and multifaceted aspect of the deck make it difficult for your opponent game one to remove the correct stuff. For example, removal on Delver of Secrets and a countered Lingering Souls isn't a huge deal since it likely allows you to not worry about your Notion Thief being removed when you flash it in during their end step.
I haven't tested the Vendilion Clique yet, but it seems better than the past Meddling Mages that were in there before. I also haven't made a decision between Grafdigger's Cage and Relic of Progenitus yet, but both work in terms of slowing down the opponent. Additionally, Leyline of the Void might be a better option than both (since black is available) and could easily slow down delve decks and any graveyard strategies.
