Bloodmage Thalnos - Fantastic in any Mage, Shaman, Druid, or Rogue deck. Aya Blackpaw - Absolutely necessary if you want to play any jade deck. The Curator - Amazing draw for control decks that can run multiple tribes, especially Paladin.
Basically if you're aiming to build your collection over time, dusting cards, especially Legendaries, is super inefficient. If you don't have the cards for a fad deck, just play ones you've got, or substitute another card into the deck until you have earned enough gold, or received the dust through duplicates or are a or end of season, anywhere except dusting legendaries. It'll make your collection building far more efficient and the fad deck will more than likely be countered by the meta in a few weeks anyway
I'm sure you can look at all the useless Epics, Rares and Commons in your collection and come up with 400 dust there. I've done this a few times already. Trust me, you don't want to dust any of those 3...
honestly if you don't like the decks that play kun, it's not a terrible one to disenchant because the way he's used now isn't really going to change - In wild with Aviana, and in standard ramp decks. he's always going to be a fun card in those decks though.
I willnever disenchant Aya, Thalnos or the Curator. I am using all of them.
IF, AND ONLY IF, you really need the dust, I would disenchant Thalnos. Thalnos is a nice addition to some decks, but it is not as necessary as Aya in any Jade deck or The Curator in a... well, a Curator deck, as most Midrange pallies and Quest Warrior decks right now.
Personal experience: I once disenchanted my Thalnos to craft Dr. Boom, I regreted it later, not because Doom wasn't super good, but because I wanted Thalnos back in my freeze mage decks, then I had a lost of 1200 dust. :c
If you don't care about your overall collection and if you are 100% sure that you never want to play jade decks (keep in mind that wild may be better supported by blizzard in the future) than Aya can be disentchanted but only small additions of a few more jade cards (like jade claw and jade lightning) allready make her extremly powerfull.
Thalnos may be the overall most usefull classic legendary. Even if you don't care about your collection never disentchant such usefull evergreen legendaries. Since release there was no meta without some decks which included Thalnos.
Like Aya, the Curator isn't in the evergreen set so theoretically it could be disentchanted but the big downside in doing so is, that this is a card which can be used in nearly all styles of playing (not in all decks!). It is key in some aggressive midrange decks like the current midrange Paladin and it also makes slower controll decks like taunt warrior way more consistent. I predict that this card will be used in the next 2 expansions in new decks too.
Kun is a fun but not really necessary card in my opinion. If you really need dust this one is most certainly the best one.
I recommend to don't disentchant anything like many others but if you are really hurting for dust (and if you are sure to never play jade) I would disentchant them in this order: Kun - Aya - Curator - Thalnos