A recently spoiled card from Khans of Tarkir shows a new mechanic for the Jeskai called Prowess.
Basically, whenever you cast a noncreature spell any creatures you have with Prowess get +1 power and +1 toughness until the end of the turn. It’s a simple combat trick, but 1 power and toughness can go along way towards swaying a battle in your favor. For example, if you have Jesaki Elder that is blocked by a Runeclaw Bear and want your Jeskai Elder to survive, all you have to do is cast a noncreature spell and all of a sudden that 1/2 creature becomes a 2/3, which is just enough to kill the Runeclaw Bear without dying. While a simple green combat trick like Giant Growth would accomplish the same result, Prowess allows players to use spells other than combat tricks to buff their creatures, which is a huge boon to the versatility of a Jeskai deck.
Flavor-wise, I think Prowess works really well for what I imagined from the Jeskai, since they look like a mix between monks and wizards; having spells make them stronger makes perfect sense and seems to work well in Blue, White and Red.
In terms of standard viability come the release of Khans of Tarkir, I can see Prowess being useful in a Red, White heroic deck, as the heroic triggers will also trigger the Prowess of the cards from Khans. We’ll have to see more cards in KoT to really say whether Prowess will be a bit game-changer in Standard, but the synergy between Theros’ heroic mechanic and prowess is blatant.
Overall, I’m a big fan of Prowess, as it gives the Jeskai something that will benefit them greatly, as Blue, Red and White have a ton of spells to work with. It also fits their flavor perfectly. We’ll see how it stands up against the other mechanics from Khans of Tarkir as spoiler season comes around the bend.