It is no secret that of all of Overwatch's Heroes, Mercy is one of the most popular to play as, partially due to her ease of use, partially out of practicality, and partially because she has one of the most powerful set of abilities available in the game. With the recent Junkertown update, however, the queen of healing has received a rather significant set of tweaks to her abilities. To make a long story short, Mercy will no longer be able to revive more than one teammate at a time, her revive ability is now on a 30 second timer, and her Ultimate ability has been changed so that upon activation, she can fly freely, heal multiple people at once as long as they are within range of each other, revive someone once every 10 seconds, and generally be harder to kill for 20 seconds. In theory, such a rework would help establish a larger divide between highly skilled Mercy players and their lesser skilled brethren, but whether or not this is going to be true in practice remains to be seen.
In any case, Mercy's rework doesn't really affect her counters and the kind of teams that she works well with. Flankers like Genji and Tracer are still hard counters to Mercy, and you're not going to have an easy time if your team insists on having some super high mobility team composition that's spread out all over the map, so that should be taken into consideration if there is a Mercy on either team. If anything, it is now much easier to play against a team that has a Mercy on it because you don't have to worry about her reviving an entire team after you wipe them out with a couple of Ultimate abilities. On the flipside, it might be marginally easier to defend your own team's Mercy simply because she should be able to fly away from danger every once in a while, though that doesn't mean that you can completely ignore the enemy team while they go around your frontline. Keep in mind that because Mercy can no longer help a team recover from a complete wipe, it may also be a good idea to always have a secondary healer like Lucio or Zenyatta to protect your team from some of Overwatch's more devastating Ultimate abilities.
Of course, the real question on everyone's mind is what kind of impact will Mercy's reworked abilities have on your average match, especially if you have to play as her. First and foremost, her Resurrect ability only has a 5 meter range, and its 30 second cooldown is not reset upon death. As a general rule of thumb, you may thus want to save Resurrect just in case some of your more important Heroes fall in battle, with Tanks ideally taking priority over all other dead Heroes. That being said, whoever you deem to be the most important in a given situation may vary wildly, as Resurrecting a dead Reinhardt who is almost certainly going to instantly die again may not give you as much value as Resurrecting a dead Zenyatta who is ready to cast Transcendence. Communication with your team is key here, as it always is in Overwatch.
Furthermore, it should be noted that Mercy's new Ultimate ability, now called Valkyrie, is much more of a proactive ability than it was before. The most ideal situation to cast it is when either team is making a push and someone on your team died so that you can Resurrect them and then instantly refresh the cooldown on Resurrect by casting Valkyrie. By doing so, you can revive two people in no time at all, potentially giving your team a numerical advantage. Casting Valkyrie will also let you heal multiple targets at once, which should be a significant incentive for your team to stay somewhat close together as Mercy can outheal the damage that a lot of Heroes can do. While Valkyrie does improve Mercy's sidearm to some degree, it should not be used in an offensive manner; dealing 20 damage per shot while flying all over the place may sound fun, but it is nowhere near as efficient as being able to heal multiple Heroes at a rate of 60 health per second per teammate.
In addition, just because you can fly, doesn't mean that you are invincible (if that were the case, Pharah would be unstoppable). If the other team lacks the ability to deal consistent damage at range, then sure, use Valkyrie to fly above your teammates and act as a kind of a spy satellite at the same time by spotting enemies and calling out their locations, but if the other team has every hitscan Hero in Overwatch, you should probably just float around a corner of a building or somesuch to avoid getting shot out of the sky. Speaking of movement options, it might pay off to learn how to utilize Guardian Angel and Angelic Descent in tandem (as your momentum is now preserved while using these two abilities), as being able to maximize the efficiency of Guardian Angel's cooldown gives you more time to stay mobile, helping you and your team stay alive.
There is a negative side effect of Mercy's rework unfortunately, and it's not so much a problem with the rework itself as it is a problem with how people generally play Overwatch. You can't expect someone to be a solo healer as Mercy anymore without taking into consideration whether or not they can see you from where they are, because line of sight is so much more vital now to utilize Valkyrie to its fullest extent; even at its most basic level, being able to maintain line of sight to your healer makes it easier to defend them. Similarly, the concept of picking the right Heroes, watching the spacing between teammates, and playing less recklessly is one that everyone should keep in the back of their mind as magical, game-saving multi-person revives are no longer an option, especially if your team is incapable of protecting the healer properly. After all, Lucio can, with practice, heal about as much as Mercy can, without being as cripplingly reliant on the competence of the rest of the team. Remember, as odd or pretentious as it may seem, there is some truth to the saying "If your healer is having a problem, then everyone is having a problem."