Reckless Bushwhacker (2R)
 Creature - Goblin Warrior Ally
 Surge {1R} (You may cast this spell for its surge cost if you or a teammate has cast another spell this turn.)
Haste
When Reckelss Bushwhacker enters the battlefield, if its surge cost was paid, other creatures you control get +1/+0 and gain haste until end of turn.

Deepfoam Skulker (5U)
Creature - Eldrazi
Devoid (This card has no color.)
Whenever a creature you control deals combat damage to a player, you may draw a card.
{3C}: Target creature can't be blocked this turn.

Dread Defiler (6B)
Creature - Eldrazi
Devoid (This card has no color.)
{3C}, Exile a creature card from your graveyard: Target opponent loses life equal to the exiled card's power.
It left behind an echoing emptiness more horrible than the grave.

So far, this has got to be my most favorite card in the set. Imagine putting the Dread Defiler in a heavy-beatdown deck, for example with a green-black demons deck? Imagine pairing this with something like Doomgape.

Stone Haven Outfitter (1W)
Creature - Kor Artificer Ally
Equipped creatures you control get +1/+1.
Whenever an equipped creature you control dies, draw a card.
The makers and menders are the first called to battle and the last to leave.

This only hints at an equipment-ally intro deck. I guarantee it. 

For Oath of the Gatewatch, Wizards have made quite the interesting mechanics, including of course, the potentially-controversial, colorless mana.  Here are the following mechanics, starting with the big gun ( in my opinion): the new basic land, and its new mana color ( or, no color? )

Colorless Mana

This symbol means it is colorless mana, a sixth kind of mana ( seventh if you count snow). It is different from generic mana since colorless is a sub-type of generic mana; basically, generic mana, now, can be paid with either red, green, blue, black, white, or colorless mana. This of course goes very well with the 'colorless' theme of the Eldrazi, and the Devoid mechanic introduced in Battle for Zendikar.

The cost {C} can only be paid with colorless mana, so if you build a deck around colorless spells, with a fusion of colored ones, then for example, there would be such a term as " I have a red-colorless deck, or blue-colorless control deck". This addition ripples throughout new and existing cards. For example:

... in this print of Uknown Shores, the first ability adds {C} to the mana pool. You can in turn, use this to pay generic mana costs. The second ability has an additional cost, which is 1 generic mana, which can be payed with any colorless mana or with colorless mana. Another example would be the Sol Ring, which instead of producing 2 generic mana ( which by now sounds awfully ridiculous), which was actually a misinterpretation of the ability, actually produces 2 colorless mana. Which can still be payed for generic mana costs or colorless costs themselves.

Oh and by the way, here is the new land:

Important note: see how it doesn't have a basic land type? Unlike with a forest, which says Basic Land - Forest, Wastes can't be selected when a card instructs you to choose a basic land type. This normally means some number of fetch-effects on both creatures and non-creatures.

So technically, two things happen: first, it will be indicated in the mana cost if it requires generic or colorless mana ( as there is now a new mana symbol), and two, whenever you see the generic mana symbol on an old card's ability ( old meaning pre-oath), e.g. Sol Ring, it means it produces colorless mana, and not generic mana, as it cannot produce all 5 colors at once with just one symbol. If that makes sense.

This makes things somewhat complicated at first, especially for me, as I am a young and intermediate-level planeswalker, only beginning to master the basics. 

Cohort

 Cohort is an ability that needs two allies to work. The cost is always the same: {T}, Tap an untapped Ally you control. The effects vary with each card. Remember, unless the main ally ( the one whose Cohort ability you wish to activate) has ally or has a way to remove summoning sickness, its ability cannot be activated on the same turn you cast it. 

Munda's Vanguard reminds me of the Phalanx Leader from the Theros Block. Imagine if you can trigger this twice or thrice in a turn! Most optimally in some sort of allies-tokens deck.

The next mechanic reminds me of Bolster.

Support

Support boosts creatures while supplying additional effects ( as far as the cards that have been spoiled go). Support N means putting a +1/+1 counter on up to N target creatures. Keyword 'up to', meaning you can choose less than the indicated number, but regardless of the number of targets, each target would still get only one +1/+1 counter on it. Support may also have no targets. On a sorcery (or instant) this can target any creature. But on a creature itself ( or another permanent), support must target other creatures. 

Like this guy:

Looks like we might see a number of cards that benefit from +1/+1 counters. Proliferate might be a thing again. Heck Proliferate might be really good in this format. Bolster-Support-Proliferate decks. Now that I think about it, another good card that has great synergy with Support is the Enduring Scalelord, which gets a +1/+1  counter whenever a   +1/+1 counter is put on a creature: so Support 2 gives the Scalelord 2 counters, and Support 6 gives the Scalelord 6 counters!

The last mechanic for the Oath is:

Surge

Surge is a nice way to get a discount on mana costs, kind of like the opposite of the Kick mechanic introduced long, long ago. Some cards might even have additional effects for being Surged, just like being Kicked.

The reduced mana cost can only be payed if you( or a teammate!) cast another spell this turn, regardless if that spell was countered, resolved, or still in the stack. For example, if you surge the Tyrant, not only do you get a 5/4 flyer for 5, but you also get the ability to bolt a creature or player!

Just a quick thought I got from seeing this card: play Dragonlord Kolaghan first, then the Tyrant. Damage overload if your opponent can't defend against flyers.

Well, that's it. Those are the mechanics for the Oath of the Gatewatch. These might get even more powerful, depending on which cards will be in the set, so stay tuned for that!

Sphinx of the Final Word (5UU)
Creature - Sphinx
Sphinx of the Final Word can't be countered.
Flying, hexproof
Instant and sorcery spells you control can't be countered by spells or abilities.

He answeres questions as readily as he asks them, but his answer is always "no".

Crush of Tentacles (4UU)
Sorcery
Surge {3UU} (You may cast this spell for its surge cost if you or a teammate has cast another spell this turn.)
Return all nonland permanents to their owner's hands. If Crush of Tentacles's surge cost was paid, put an 8/8 blue Octopus creature token onto the battlefield.

ENDBRINGER (5C)
Creature - Eldrazi
Untap Endbringer during each other player's untap step.
{T}: Endbringer deals 1 damage to target creature or player.
{C, T}: Target creature can't attack or block this turn.
{CC, T}: Draw a card.

Kozilek, the Great Distortion (8CC)
Legendary Creature - Eldrazi
When you cast Kozilek, the Great Distortion, if you have fewer than seven cards in hand, draw cards equal to the difference.
Menace
Discard a card with converted mana cost X: Counter target spell with converted mana cost X.

A void as cryptic as reality itself.

Mina and Denn, Wildborn (2RG)
Legendary Creature - Elf Ally
You may play and additional land on each of your turns.
RG, Return a land you control to its owner's hand: Target creature gains trample until end of turn.

Greenweaver Mina and her twin brother Denn lead the last of the Mul Daya tribe into their uncertain future.

 Ayli, Eternal Pilgrim (WB)
Legendary Creature - Kor Cleric
Deathtouch
{1}, Sacrifice another creature: You gain life equal to the sacrificed creature's toughness.

{1WB}, Sacrifice another creature: Exile target nonland permanent. Activate this ability only if you have at least 10 or more life than your starting life total.

Nissa, Voice of Zendikar (1GG)
Planeswalker - Nissa
{+1}: Put a 0/1green Plant creature token onto the battlefield.
{-2}: Put a +1/+1 counter on each creature you control.
{-7}: You gain X life and draw X cards, where X is the number of lands you control.
{3}

Kalitas, Traitor of Ghet (2BB)
Legendary Creature - Vampire Warrior
Lifelink
If a nontoken creature an opponent controls would die, instead exile that card and put a 2/2 black Zombie creature token onto the battlefield.
{2B}, Sacrifice another Vampire or Zomvi: Put two +1/+1 counters on Kalitas, Traitor of Ghet.

Chandra, Flamecaller (4RR)
Planeswalker - Chandra
{+1}: Put two 3/1 red Elemetal creature tokens with haste onto the attlefield. Exile them at the beginning of the next end step.
{0}: Discard all the cards in your hand, then draw that many cards plus one.
{-X}: Chandra, Flamecaller deals X damage to each creature.

Goblin Dark-Dwelllers (3RR)
Creature - Goblin
Menace

When Goblin Dark-Dwellers enters the battlefield, you may cast target instant or sorcery card with converted mana cost 3 or less from your graveyard without paying its mana cost. If that card would be put into your graveyard this turn, exile it instead.

Kozilek's Return (2R)
Instant 
Devoid (This card has no color.)
Kozilek's Return deals 2 damage to each creature.

Whenever you cat an Eldrazi creature spell with converted mana cost7 or greater, you may exile Kozilek's Return from your Graveyard. If you do, Kozilek's Return deals 5 damage to each creature.