Three new cards came out for the new Hearthstone set that are very interesting indeed. They all have a similar style - they cost zero, but when cast they use up all of your mana to do something. There is a healing effect that heals for double the mana used, a summon that creates a random minion of the same cost as the mana used, and a damage spell that does damage equal to the mana used to a minion only. They look like this:
Keeper can be used as a silence, sure, but it can also drop 2 damage to achieve lethal, or just blow up a 3/2 minion. Spellbreaker is junk because it lacks the flexibility the Keeper has even though its raw power is plenty high enough. When you need a silence paying 4 mana for it and getting a 4/3 body is just fine. The problem is that you will often be holding Spellbreaker when you don't need a silence and he will be a big steaming pile of crap. This makes it abundantly clear that a card that can flex depending on your needs at the time is hugely more powerful than the raw numbers might indicate.
This is why I am definitely excited about these new flexible cards. When you face a Hunter you can just use the summon on turn 1 to get some random 1 cost dork to fight the opposing 1 drop. When facing a control Warrior you can save it up to use on turn 10 to get some giant monster they have to deal with, adding one more enormous threat to your deck. When facing a tempo deck you are never going to look at your hand and cry because you have nothing to do on turn 3 since you will always be able to just pop out a random 3 drop. It won't be great, but a lot of the time all you need is *something*, not the perfect thing.
The damage and healing spells are similar. Sure, spending turn 10 healing for 20 health isn't usually what you want to do, but against a Hunter it is likely a game winning move. Unless you have completely lost control of the game they are going to be trying to finish you off and 20 extra health is probably an insurmountable lead. Just as important is the ability to heal for 2 or 4 with 1 or 2 mana. There are tons of times where spending a card and 2 mana to heal for 4 is a great play but you can't afford to put that card in your deck because of how narrow it is. Being able to sneak in a heal on a medium size minion to maintain board position is sometimes really useful, and when that card can be used in the late game for a massive heal instead it is going to be a great option to have.
These flexible cards are rarely going to feel great. Nobody is going to have them listed on their "God draw" lists, and you won't be dreaming about how you are going to wreck people with slightly subpar effects. However, you also will have a much smaller chance of looking at a hand with a 2 drop and a bunch of 4 drops and being super sad that turn 3 is a bust. Any hand with a single flexible card in it will be much more resistant to that sort of thing, and much more able to turn a card advantage into a board advantage by maximizing mana spent every turn. Just like Keeper of the Grove, nothing these flexible cards do is all that insane (barring the minion one producing a King Mukla every so often, to be fair) but the ability to always have something good to do is not to be ignored.