Leeroy Jenkins + Ancestral Spirit + Reincarnate
While Naxxramas has done a lot to slow down the burst-from-hand damage that was prevalent in recent seasons, it has also enabled some new wombo combos. Most notably, Shaman now has its own pseudo-Shadowstep in the form of Reincarnate. Between Ancestral Spirit and Reincarnate, you can bust through a 6-health Taunt and still deliver 12 damage to your opponent's face, possibly adding even more pain with the inclusion of a second Reincarnate or some Rockbiter Weapons.
Shaman wasn't a stranger to burst combos before, often utilizing Al'Akir with Rockbiters and Flametongue Totems to devastating effect, or Leeroy with Windfury. The difference now is that Reincarnate is actually a very flexible card when placed in a deck designed around it. Throw in Sylvanas, Cairne or some Earth Elementals, and it becomes ever less likely that Reincarnate will end up stuck in your hand while you wait to draw a second combo piece. In any card game, you want to minimize the risk of dead draws, which in contrast Windfury often was.
Stalagg + Shadowflame
Again, not a complex combo, but it's worth discussing just how good Feugen and Stalagg are in Handlock. The deck used to have gaps in its curve, particularly at the 5 and 6 Mana slots. You could always play a Mountain Giant on turn 5, but that was more or less your only mana-efficient play. Nowadays people are experimenting with Sludge Belcher, but I don't rate the card as highly others seem to. Instead, I've opted to toss in these two along with Loatheb, cutting some of the more situational older cards.
Taunt up Feugen with a Defender of Argus or a Sunfury Protector and he provides an excellent wall behind which to hide your face, and your giants. Stalagg, meanwhile, is the perfect card to use Shadowflame on for a major board clear. Not only does 7 damage kill just about everything your opponent is likely to have, his death isn't even a downside. It only brings Thaddius closer. Besides, the deck is all about playing huge stuff at a discount. And the only thing bigger than Thaddius is Deathwing.
#3: Poison Seeds + Starfall
This is certainly an expensive combo to pull off, costing you 9 mana and two cards, but the effect is powerful enough to consider. The combo's main strength lies in its potential to preserve some board presence while guaranteeing a wipe of your opponent's side (barring potential Deathrattles from stuff like Voidcaller).
The impact of Naxxramas has been to refocus the game on board control, so having the tools to sweep away opposing minions is now more integral in deck construction than ever, and this is certainly one of the best full clears.
Of course, it comes with a significant risk attached. A 9-mana combo is a significant investment and your dudes get turned into Treants, so you have to consider whether the combo pieces also fit into your deck as standalones in order to minimize any over-reliance on their synergistic effect.
#2: Mad Scientist + Duplicate
In my initial evaluation of the new class cards , I came down pretty hard on Duplicate, and I'm still of the opinion that it's mostly terrible. But this combo is actually pretty interesting because in a list stocked with other Secrets it allows for efficient deck-thinning.
Mad Scientist is both cheap enough, and efficient enough, that it should enable you to churn through your Secrets quickly and effectively, particularly when used in conjunction with other Secret-centric cards like Kirin Tor Mage and Ethereal Arcanist.
In a sense, the Scientist is like a slightly better Loot Hoarder, except you know what kind of card you're going to draw, and you get to play it for free. It seems like Secrets Mage has received quite a boon with Mad Scientist, and is now a pretty respectable deck archetype instead n of the silly gimmick it was previously.
Baron Rivendare + Sylvanas Windrunner
Baron Rivendare is arguably the key thematic card in the Naxxramas set. There are too many possibilities to list here, but it's a card that can generate immense amounts of value from a good board position, has intense flexibility, and is all death-y. It can enable you to flood the board, clear the board, manipulate the board, buff the board...
My personal favorite, though, is playing him in tandem with Sylvanas for a 3:1 trade whenever she can be used to kill a minion while having two remaining targets for her double mind control. It's just such a devastating turn against any deck that it's hard, if not impossible, for them to recover from. So far, after the Echoing Ooze, Rivendare has been my favorite of the new cards to play with, and I can't see that changing anytime soon.
I hope you enjoyed checking out these combos, and maybe they've given you a couple of new ideas for your own decks. Good luck on the Ladder. It's certainly hectic out there!