There are now 280 Spell craft cards available for building deck. Most are extremely well designed, but not all get equal attention. This thread is intended to try to identify some of those cards.
Let me begin with one of my favorites: the deepwood fey (forest). At one power point casting cost, no card will not be single-handedly overwhelming. But a card that removes all types of immunities (even if to a single randomly chosen opponent) can be very valuable -- especially in a deck that could otherwise struggle with immunities. Unlike null wand (air), deepwood fey is not bad even when the opponent has no immunities. Its quick trait gives it a chance of inflicting at least one point of life damage to an opponent. Its evasive trait allows it to move away from a threat -- or into the path of one when a sacrificial blocker is needed. And the fact that it is not magic immune allows it to work in combination with spells like living bomb or bodyswap.
Another nice card often overlooked is the ominous egg (underearth). Because it is a barrier, it may not be seen as a dangerous choice, but I think quite the contrary. If the low cost minions in a deck are limited to those that are dangerous, but hard to play in a congested board (think aquamancer or toxipede), the ominous egg is the ideal vehicle to bring them into play. As the egg is usually triggered during the opponent's attack phase, the creature inside will get to use its special before facing the attack of an opposing unit. Eggs can also grant a power point advantage. If your deck contains no cost 0,1, or 2 minions, for the 1 cost of the egg, you bring into play a cost 3 minion (provided you can force your opponent to destroy it -- usually not too difficult). Finally, the egg is excellent defensively because it must effectively be destroyed twice. Need to block an enemy dragon while that enemy has burning world in play? Don't use a fey spirit -- it is destroyed by the aura before the enemy attacks. Need to block a deepwood ash? Don't use a lava imp -- the imp is destroyed first by the ash's power. But an ominous egg works great in both cases. And if you want a blood orb sacrifice, you can't use a creature to block something stronger and sacrifice it later. But you can use an ominous egg.
The primeval flame card (fire) is another often overlooked gem. Barriers in general are not often utilized in PVP -- believe because most players consider them too passive. Reconsider about the primeval flame. It's special ability (inflicting one damage to an enemy of choice) is akin to the aquamancer (although the primal flame loses health in the process). But the primal flame also affects barriers, and is magic immune. It's strength (2) prevents nearly all opponents from destroying it in a single hit as well. Thus, it is far less vulnerable than an aquamancer -- especially in later game stages. Primeval flames are particularly devastating when combined with wild growth or living essence which restore its health
If you have never tried a triton pearl diver (ocean), it is worth doing so -- if you have a deck with lots of expendable items (such as pearls or lost spellstones). The diver automatically restores one item from discard to hand every turn. It remains free to act -- which often means running from a stronger opposing enemy. At its worst, the pearldiver is a cheap sacrifice to block a dangerous enemy minion for a round -- about all any cost 1 card can do when actually opposed.
The effect of restoring used items can be very impactful, and just one pearldiver in play can draw a lot pretty quickly.
Another card I like a lot is the undead triton (swamp). At 2 cost, I consider it a disposable blocker for nasty enemy minions. But it has a lasting effect of draining enemy strength. It thus blocks a dangerous minion for 1 round, while softening said enemy for my big minions next round. But it is capable of indefinitely blocking an original strength 3 enemy or defeating a strength 2 enemy. Head to head, it defeats most equal cost cards, and neutralizes a large number of more powerful cards. To me, this is a must have for most swamp decks.
So far I have odentified mainly minions (and bariers), msinly because I use them heavily. Bit several spells also merit comment.
Astral shield (air), which grants magic immunity is definitely useful in the right context. One of my biggest fears with growing deep spawn or deepsea thing is my opponent playing slavers or bodyswap so I have to face my own creation. The biggest vulnerability of aquamancers is the direct damage spell. The biggest inhibitor of catastrophe is the destruction of my own units. Simple magic immunity eliminates a myriad of problems -- including taken under, meteor and sink. Just be aware casting in on combat immune creatures removes combat immunity.
Fire's lavapult is also an excellent, low cost card. By inflicting one point of damage to any enemy on its path, it renders a lane almost uninhabitable to your opponent -- while not impeding you at all. It is especially nice in combo with siren -- the siren draws cards into the lane; the lavapult kills them so other cards can be drawn to the siren. But watch out if your opponent has fire faction -- one pyrohydra and the card backfires badly.