Concerted Effort is a 4-mana Enchantment that offers a rather unique effect. At the beginning of each player's upkeep (not just your own), all creatures you control gain flying until end of turn if a creature you control has flying. But it doesn't stop there. The effect is repeated for fear, first strike, double strike, landwalk, protection, trample, and vigilance.
Let’s say you have Aurelia, the Warleader on board. Each turn, all other creatures you control will gain flying and vigilance. Plus, Aurelia and all other creatures you control will gain any other abilities that your other creatures may have - such as the double strike of Kalemne, Disciple of Iroas.
The Commander deck that Concerted Effort often finds its way into is Odric, Lunarch Marshal. This is because the second Legendary incarnation of Odric, Master Tactician has a very similar ability to this Enchantment. However, this Odric also considers hexproof and indestructible, and some other new mechanics such as menace and skulk. So, obviously, this Enchantment offers some useful redundancy.
Another Commander who typically runs Concerted Effort in her arsenal is Radiant, Archangel, a mono-White Angel Tribal Commander. Angels tend to have a wide array of abilities. If you have a creature like Akroma, Angel of Wrath on the board, not only will all your creatures have flying and vigilance, but they'll gain first strike, and protection from both black and red.
Aurelia, the Warleader, Iroas, God of Victory, Kemba, Kha Regent, Gisela, Blade of Goldnight, Daxos of Meletis and Tajic, Blade of the Legion are some other Commanders that will include this Enchantment in their 99.
One aspect of Concerted Effort that shouldn't be overlooked is that any Auras or Equipments that give a creature a particular ability are counted by Concerted Effort's effect. That is, as long as those creatures had them at the beginning of the turn's upkeep. Say you have a creature equipped with Sword of Feast and Famine. All of your creatures would then gain protection from Black and Green. It is important, especially with Protection, to make sure that the Effort's ability doesn't backfire on you.
One particularly fun thing to do with Concerted Effort in play is to play an Equipment such as Trailblazer's Boots, which gives a creature non-basic landwalk. With Concerted Effort, this means if your opponent controls just one non-basic land, all of your creatures will be unable to be blocked. Ouch.
It’s sufficient to say that Concerted Effort is relatively underplayed considering what it can do. It does require your deck to be build around certain abilities, though. Still, there are extremely relevant abilities that this card can copy - especially trample. It can be part of your win condition if your deck is suited for it. When building an aggressive White, White/Red, or perhaps even Green/White deck, don't overlook this Ravnica Enchantment.