I used to play a lot of freeze mage back in the Blackrock Mountain expansion. When I wasn't playing I was watching Forsen play the deck. This deck resembles old freeze mage a lot. It only needs to draw and stall long enough for the combo to work.
The aim of the deck is to draw Plot Twist and Hemet, Jungle Hunter as fast as possible. Then use Hemet to clean out the deck allowing for the rest of the combo cards to be drawn. The Soularium is a nice follow up card to empty your deck after nuking it with Hemet.
Having Hemet, Jungle Hunter in hand without Plot Twist renders Hemet useless. He will nuke it since it is only a 2-cost card. That's why the deck has two Plot Twist. Always make sure to have one in hand when nuking.
Doomsayer deserves a mention because it is the king of stalling. Against most aggro decks it buys you a turn because they either skip their turn or spent it killing Doomsayer. I even had a few priests put it back in my deck.. thanks! Don't be afraid to dump it on an empty board or along with Runic Egg, Bloodmage Thalnos or Loot Hoarder. Most of your minions are purely for draw and their actual presence on the board means nothing. Be happy when your opponent starts picking off your minions, gifting you time. Besides your minions can be used to setup Defile or Plague of Flames.
Why almost no healing?
When do you need healing: When you know you are about to die. Wouldn't you rather just win instead? 30 health is a resource to be used. We are Warlocks after all.. Most cards that we can use for healing does only that and costs a lot of mana resulting in a wasted turn. Personally I'd rather hero power for a card that might clear the board or be the winning card. Rarely / never have I found myself sitting with all combo-cards in hand ready to play them but needed to heal. Drawing healing when you don't need it is the most wasteful draw in the history of draws.
Mulligan for draw cards and removal. Only keep healing and combo cards if you are facing Demon Hunter. With a bit of luck and The Coin you can pull the combo off on turn 7. On turn 6 play Hemet, Jungle Hunter. Then on turn 7 play The Coin then Dollmaster Dorian then Plot Twist then Cataclysm. Kael'thas Sunstrider makes Cataclysm (the 3rd spell) free to cast.
In case you don't know the combo
Draw your entire deck. Or just nuke it with Hemet.
Have in hand (you can have other cards among but these are the combo):
Play Coin or 1-cost or 2-cost or 3-cost spell
Play Dollmaster Dorian (5 Mana Cost)
Play Plot Twist (2 Mana Cost)
Win because board, deck, and hand is empty.
Why not this or that card?
Reno Jackson or Antique Healbot is used in different flavours of this deck. They present great heal but aren't killed off by Hemet, Jungle Hunter. This can slow down the deck by several turns. Getting Hemet off means you want as few cards as possible to remain in your deck. In my opinion cutting out Reno and playing to Hemet's strengths is what makes this deck tier 1.
Valdris Felgorge is not needed because the 10 card handsize is plenty enough. Plus it gets in the way of Hemet.