Blizzard is Overhauling Hearthstone to Create a Balance
Blizzard is taking precautions against the inevitable 'power creep' which evolving card games often face, overhauling Hearthstone with separate modes for different deck builds, as well as additional slots for new decks.
To put it simply: Blizzard has separated Hearthstone's multiplayer games into two modes. The new default competitive game mode, Standard Mode, will require players to use decks built only from cards released in the "current and previous calendar year", alongside Basic & Classic packs. If you're unsure, that includes Blackrock Mountain, The Grand Tournament, and League of Explorers.
If you want to keep using cards from Naxxramas and Goblins vs. Gnomes, you'll have to use Wild Mode. For full details, be sure to check out Blizzard's latest blog post explaining the ins and outs of the system.
Additionally, if you're interested in game development and want to learn about 'power creep' and its effect on Hearthstone and gaming in general, why not check out Extra Credits?