I will list the cards I think have had a major impact in Modern printed in 2015. This means no love for Den Protector, Deathmist Raptor, Hangarback Walker, Dragonlord Ojutai and Silumgar's Scorn, although some of them have been tried in some Modern decks with moderate success. It might be curious to revisit this list in a year; who would have guessed Keranos, God of Storms would have become such an important card. Let's go in ascending order.
Honorable mention: Harbinger of the Tides
I hadn't thought of including this card in my list until I checked the list of most played creatures of the format at mtggoldfish and saw it in such a high position. The combination of Harbinger with Aether Vial gives Merfolks an instant removal spell that also counts as a creature and contributes with two blue mana symbols to the devotion of Master of Waves.
#10: Painful Truths
This card hasn't make an impact yet in Modern, but it's starting to see play in the sideboard of several mid-range decks and even some Twin versions. Drawing three cards is still a very powerful effect in Modern, and Wizards has found the optimal cost after the failed experiment of Treasure Cruise. Three mana are the maximum we are willing to pay for this, and 3 life seem easier to pay than a fourth mana to cast cards like Concentrate and Harmonize.
While Elves are still less played than Merfolks, they've improved more since Origins came out, and this card is responsible for most of the improvement. Elves lack haste to surprise any opponent when playing an aggro plan, while the pure combo versions are too fragile due to the amount of removal of the format. However, Shaman of the Pack allows you to deal damage with your Elves before you get to attack with them, making the aggro plan more consistent and the deck more competitive.
Dragons of Tarkir brought a cycle of color based hate cards, from which only Rending Volley has become a format staple, replacing Combust as the favorite Twin hoser, although unable to kill Siege Rhino or Baneslayer Angel.
We already knew from Tombstalker that delve is a very powerful mechanic, and Treasure Cruise and Dig Through Time certainly proved the point. Despite being just a common, Gurmag Angler has supposed an upgrade to the Future Sight demon, because costing just one mana is better than costing two, and Modern is filled of flying chumpblockers. Patrick Chapin tried to break the card since he saw it printed and momentarily succeeded with his Grixis deck, now obsolete. Gurmag Angler has also seen play in Dredgevine decks.
This card looks like a worse version of Siege-Gang Commander, but it's actually better and I'm still surprised of its low price, probably because it's only played in Modern. Chandra's parents make the difference against mid-range decks, and are also a fantastic card against Affinity. Comparing them to Lingering Souls is a fairer comparison.
It saddens me to see Burn getting new toys in a yearly basis, but 2015 has given red mages the ultimate tool for their decks. Life gain will always be a problem for Burn, and Skullcrack alone can't deal with it. A card like Flames of the Blood Hand is too expensive, while Rain of Gore is only sideboard material and requires a splash to an extra color. Atarka's Command solves the problem, while also being able to give +2/+2 to a horde of attacking Monastery Swiftspears.
The newest Jace is responsible of the most expensive Standard format ever, and has been played even in Vintage. However, even without the Mind Sculptor in the format, it hasn't become a dominant force in Modern yet. So far, Jace has displaced Gurmag Angler as the card to build around your Grixis deck, in which it might be even more powerful than Snapcaster Mage, but still lacks the flexibility of the Innistrad card to appear in more archetypes.
The banning of Birthing Pod was one of the most controversial decisions of the year. Many players cried out loud, signed online petitions and called for demonstrations. However, just a few months later, nobody misses the artifact from New Phyrexia. Collected Company doesn't just have a sedative effect in the community, but has allowed the Melira combo to be still viable in Modern, has revived Big Zoo and gave Elves its first push towards being a competitive deck. Collected Company offers these decks card advantage, resilience to removal and can even get some game winning combos into play at instant speed.
Gurmag Angler's best friend has some advantages over the big fish that make for the lacking point of power; a lower converted mana cost and a relevant ability for the late game are factors that don't force you to go all in filling the graveyard to get a fatty. Tasigur has been played in almost any black mid-range deck, even replacing Dark Confidant in Junk, and also in control decks. It's been also one of the favorite alternative win conditions of Twin players. It has made Jund players cut some Abrupt Decays in favor of Terminates, and has made Doom Blade and Slaughter Pact unplayable cards in Modern.
2015 started with Junk being the most popular fair deck and ends with Grixis being the most popular fair color combination. The main reason is the combination Snapcaster Mage + Kolaghan's Command. The same way playing without blue delve spells a year ago meant playing an underpowered deck, playing a fair deck without this combination of cards is starting to seem equally wrong. They can be found in the most diverse decks: with Delver of Secrets, with bigger beaters, with Jace Vryn's Prodigy or with Splinter Twin. Kolaghan's Command is a really versatile spell. When analyzing the card at first, many people just looked at the worst possible case, in which it's just a worse Blightning, forgetting so many amazing situations. It has turned Batterskull and equipment in general (outside of Cranial Plating) unplayable in Modern; those are big words that deserve the first place of my list.
Is there any card I've missed? Feel free to point it out. Will this list look the same in a year? We'll see then.