Hearthstone is releasing a balance change update that sees nerfs to some Classic / Basic cards. It is an unexpected change, but it seems like the bigger problem is the Odd and Even Cards, Genn and Baku which are limiting design space (for both new cards and old).
Nonetheless, here are the cards that are getting nerfed. The changes will release on 5 February, so you will get dust refund for the cards (except the basic ones). Check out Hearthstone's blog for more.
#1. Cold Blood
Cold Blood will now cost 2 mana, up from 1. It was a staple in Odd Rogue which allowed small, aggressive minions and Leeroy Jenkins to get a huge damage burst.
#2. Flametongue Totem
This card saw play in many aggressive Shaman decks and is a basic card, so you won't get any dust refund for this. Flametongue Totem also saw much more play in the more aggressive Even Shaman. Mana cost is now 3, up from 2.
#3. Equality
Equality now costs 4, up from 2. This is a great way to weaken your opponent's minions and a sure fire way to destroy the board with 4 mana with Wild Pyromancer. Well, that will cost 6 mana to do now. This is a big blow to Paladin, especially in Arena.
#4. Hunter's Mark
Hunter's Mark got nerfed, again. It used to cost 0 mana for the longest time until they changed it to 1. Now it costs 2 mana so it is harder to Hunter's Mark + Candleshot a minion now. This is also a basic card, so you won't see any dust refund.
#5. Emerald Spellstone
This is the only card which isn't from the basic/classic set. Emerald Spellstone will now cost 6 mana, up from 5. It is a staple in secret hunter and especially powerful in Arena. It is still pretty powerful at 6 mana.