Heroic mode Naxxramas

Wojtek addressed most of your questions, but I'm just going to add details to this one question.

**Are there certain cards that are more cost effective to DE? ** There are two things that affect how much dust you get when you disenchant a card.

• Card rarity
• Foil/non-foil

Card rarity can be found by looking at the gem in the center of the card. Common cards have a white gem and give 5 dust when you disenchant them. The colors match the colors in World of Warcraft, with the highest being orange (legendary). Cards that are more rare will disenchant into more dust (you can see how much as Wojtek explained). Also, the cards you get as you level your hero to 10 are considered "basic" cards and don't have a rarity gem. They can't be disenchanted.

-          Normal           Gold    
-          Craft    DE      Craft   DE  
Common      40      5       400     50  
Rare        100     20      800     100  
Epic        400     100     1600    400  
Legendary   1600    400     3200    1600  

If you go through the math to complete a complete set of 2 copies of every card (leaving all of the disenchanting and crafting until last), you're much more likely to need to disenchant commons and craft legendaries. source

Sometimes you can find gold (foil) versions of cards when you open packs. These cards functionally play exactly the same, but if you disenchant them (and keep the non-gold versions of the card) then you'll get more dust. On the other hand, gold versions are animated (some are QUITE nice) and a nice thing to show off.

So, to answer your question, it is generally more cost effective to disenchant the gold cards, if you don't care about the aesthetics and status of using the gold cards.

Another fact to note, if you use the "disenchant extra cards" button, it will automatically keep 2 regular and 2 gold copies of each card (for a total of 4 copies of the card). You have to manually decide which cards you want to keep and disenchant the others.

So from the looks of things, there aren't actually many Easter Eggs in Hearthstone, at least not many discovered yet. It is indeed a little surprising, but to be fair, Hearthstone isn't exactly the largest of games. The closest things to Easter Eggs are actually more like hidden references to the lore of Warcraft.

• If Cairne Bloodhoof is played against a warrior, he says the line "Garrosh, you are not fit to rule the horde" rather than his usual line.
• If Illidan Stormrage is played against a druid, he says the line "Hello, brother" rather than his usual line.
• Millhouse Manastorm, the hero in the tutorial, says a line "Just you wait until I have 10 mana!" This is a reference to the old Millhouse Manastorm card, which had "Battlecry: Put a 'Mega-Blast' card in your hand." Megablast is a 10 mana card that deals 5 damage to all enemies. Reference
• In the Hearthstone credits, there are personalized cards for team members of Hearthstone, many of which seem to reference aspects of the individual person. Reference
• The opening video has an 8 cost Pyroblast; it's now 10.
• If you click on the vegetables long enough you can get a boot.
• If you play Dire Wolf Alpha against the Big Bad Wolf, your wolf whimpers after the Big Bad Wolf scared him. Reference Video

There are of course individual cards that are references to things in World of Warcraft rather than the actual lore, one of the most obvious being Leeroy Jenkins, but I wouldn't necessarily define these kinds of things as Easter Eggs.

If individual unlisted extras are discovered, feel free to edit this question and tack them on here, so they can be easily organized. However, from what my research could find, these are pretty much the only ones discovered thusfar.