Hearthpwn

First you must ask yourself two questions: Are you ever planning to play Wild? Are you ever planning to play Classic?

If you answered yes to either of those, be very careful about your dusting decisions.

Now ask yourself some more questions: Do you care about future achievements and related rewards? Do you care about other game modes like Tavern Brawl and Duels (and possibly Mercenaries)?

If you answered yes to those, you need to be very careful about your dusting decisions.

If you answered no to all, you may dust expansion cards and Classic cards you are sure you don't want, but be aware that it will cost you if you change your mind in the future.

You cannot dust Core cards (regular or golden), and it does not matter at all. You never truly owned them, and they will always be made available to you whenever they are part of the Core set, even if they are removed one year and return the next.

Think of it like a pool hall. You may bring your own pool cue (expansion cards), but the tables and balls (Core cards) are just on loan. They will always be there for you to use, but you cannot take them with you or sell (dust) them.