Note: this article mainly concerns newer, and especially free to play players, so if you're more experienced at the game or don't mind spending a quick buck, then you can ignore this post. However, I am sure most people have made this mistake at least once.
During the Knights of the Frozen Throne expansion, my legendary from the first bunch of packs was Hadronox. I took a look at it, thought about it for a good few seconds, and immediately dusted it for a few rares. Then, less than a month later, Taunt Druid becomes a tier one deck, and I have almost all the cards necessary except for one: Hadronox, the core component that makes the deck work. Me being a budget F2P at the time, I couldn't manage to scrounge up enough dust in time to re-craft him.
Another, more recent example, during the Boomsday expansion, my buddy got Dr. Boom, Mad Genius as his free legendary. He tried in vain to make Mech Warrior work throughout Boomsday and Rastakhan's Rumble, but couldn't do it. He eventually gave up and dusted him (he hated playing Odd Warrior). And what do you know, a few expansion later, in the Rise of Shadows, Warrior has two tier 1 decks, both of which include Dr. Boom, Mad Genius. Granted, you don't need him to make the deck work, but it's just such a powerful card that you're really shooting yourself in the foot of you don't use it.
The moral of the story is: don't just dust cards willy-nilly. Suppose you opened Harrison Jones, but you REALLY need that 400 dust to make a second Academic Espionage for your Tess Rogue deck. But soon, you may regret that decision, not just because of the card you crafted, but because you don't have that legendary anymore. What if there's a new weapon-oriented deck like Kingsbane Rogue taking over the meta? You now no longer have an effective way of dealing with that deck. Many classic legendaries will constantly find their way into the meta no matter what, simply because most of them are either tech cards (e.g. Harrison Jones) or they're just good enough to put into the deck (e.g.Edwin VanCleef). And they never rotate, so they will likely find a place in some deck eventually. For a similar reason, it's good to hold onto some underwhelming expansion legendaries as well. It's often the case that Blizzard will print a legendary card one expansion, then print cards to support it in a future one (e.g. Tess Greymane and Academic Espionage), or eventually someone will figure out a deck using that legendary that happens to be very strong (e.g. Kangor's Endless Army and Mech paladin).
The worst feeling for a F2P player is being unable to make a deck because you're just missing one legendary, but you opened it before and dusted it. So I think that if you open a legendary in your pack, you should hold on to it at least for a while. Now, there are exceptions: you could just open a bad legendary like Tyrantus or Harbinger Celestia, or you could get a card for a deck that you know for sure that you won't play (I never play Priest, so I dusted my Raza the Chained and didn't regret it, even when Razakus Priest was stupidly overpowered). Worst case scenario, it's 400 dust in card form, but by dusting it you lose the ability to potentially make a deck. Think of it like having government bonds, but in the form of cards. If you don't need the dust right now, then don't dust it. The disenchant value of cards don't go down over time, so you won't lose anything for the time being.
Sorry if this has already been posted before, but I've had to suffer quite a bit because of this, so I'd rather other people learn from this mistake. Hopefully this helps someone.
