[Update: Blizzard responded to inquiries regarding drop rates. The story below has been edited to reflect this information.]
Hearthstone's next expansion will come alongside some pretty major changes to how random drops work when you open packs. The biggest change comes to Legendary cards, which will no longer give duplicates in random pack openings.
According to the announcement, Legendary drops will always select one that you don't already own. The move is meant to make it more exciting to open a Legendary, without worrying about hitting one that you've already opened or crafted. That means that once you've crafted a Legendary card, you won't open it an hour later and feeling that sting of regret. You'll also always get a Legendary within your first 10 packs when a new set comes out.
Additionally, drops won't give you any more copies of a card than you can actually use in a deck. The card limit is two for Common, Rare, and Epic cards, so you won't open three or more of any of those card types in a single pack. This happened rarely, but it was always frustrating for players to open more cards than they have any actual use for.
Blizzard confirmed to Shacknews that the drop rates for Legendaries won't change once the update goes into effect. The announcement doesn't address what happens for the rare player who already owns all the Legendary cards in a given set, but presumably they would start to get duplicates again for the dust.
The timing for the next expansion is still under wraps, but Blizzard plans to release three full-size expansions per year now. The community has expressed some unease with that frequency, since it means investing in packs that much more often to keep up. Being a little more generous with Legendary cards may address that complaint.
As a promotion, Blizzard is giving extra cards for Journey to Un'Goro pack bundles this week. You can get an extra two, five, 12, and 18 packs from the standard bundle prices. Keep in mind this is before the new changes have gone into effect, so you can still open duplicate Legendaries and three cards in a pack.