What happens when you get 500 wins with a character

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.

For Lore Purposes, i think these two cards are best looked at seperately.

Siltfin Spiritwalker is just a Murloc Shaman. Murlocs have a strong connection to the Water, as they came from the deep ocean and are just now advancing to the coast of Azeroth. Having such a strong connection to the Water, there are a lot of Murloc Shamans who use water magic to their advantage.

There is no lore connection why Thrall would be using Murlocs in battle. Infact, when he founded Durotar and Ogrimmar, Murlocs on the coast were such a strong force that he teamed up with the Darkspear Trolls to combat them.

But still because this Card is a Shaman of another race it makes sense and is inline with other class cards, like Steamwheedle Sniper or Sorcerers Apprentice.

Neptulon the Tidehunter is one of the four Elemental Lords and he rules over all Water.

Why is he a Shaman card?

Shamans command the elements and elementals for their cause, and he is the leader of all water elementals. Every being who uses water magic worships Neptulon.

Why does he give you Murlocs?

It is often speculated that Murlocs or their ancestors that lived in the deep sea were transfered to their apperance by the Old Gods like the Naga. When Neptulon still served the Old Gods, he could have been the force to do so. It is even rumored that Murlocs were send to the coast by Neptulon. Also, a lot of Murlocs worship "the Deep One" which is probably refering to Neptulon.

So the Answer is:

There is no special Connection between Murlocs and Shamans, but these cards still make sense to be Shaman Class Cards.