Also to add to Winter’s post – The difference between a successful higher key group and a nightmare group is typically not a bad tank or healer (in our guild groups, anyway), its the dps. In anything above a +2, all 3 dps are expected to perform the following tasks -while- maintaining their high dmg rotations:
-Interrupts
-Dispels
-CC
-Knowing where your ass is (aka, not pulling extra mobs)
If you are failing in one or more of these categories, it is easy to practice and get better. Plenty of the Tuesday heroic group run lower mythics on their alts and would be more than happy to let you get used to CC & also teach you positioning/paths of the various instances. Interrupts should pretty much always be on cooldown during a pull, and you should have an addon that let’s you dispel party members if you have that ability. If your interrupts require a certain pet at the expense of your dps, then yes, chances are high that you will be required to use that pet at the expense of being the top dmg dealer. Mythic+ instances, just like Mythic raiding, is about maximizing all of the tools of your class, not just the obvious one – that of doing damage.
And just as with raiding, there are mythic+ videos that will teach you the route and boss strat before you ever step inside the instance. If you do not know an instance, you are being selfish if you join a +3 group or higher. It simply boils down to this: If you couldn’t/didn’t have the time to run each instance in a 20 minute heroic pug to see the place, then I’ll be blunt, you don’t have the the time to be doing mythic+ at the expense of your group. You are actually wasting -their- time when the intention is to hit timer, but the group fails to succeed.
Lastly, if you are not taking these tasks and abilities seriously, then you will 100% be the reason 4 other people do not make timer. There is a certain amount of responsibility that you are taking on when you agree to fill a spot in a timed run. At a certain point, a mythic+ will exceed the difficulty of our raids. As of today, that point is around +3 for the normal group and +5 for the heroic group. When in doubt, you can always ask me privately where I believe you would rank in the M+ system based on your stats from the raids.
If you are wondering what is required from mid to higher level keys, you can always check out the recording of my streams on Twitch. They stay in archive for a few weeks, and you can see what makes the difference between a successful and a failed timer (and this is after the raids when fatigue sets in, lol!)
I hope that helps everyone looking to jump in, but unsure where to start. I hope everyone who wants to get into a group does, so don’t hesitate to message me about what would work best for you. =D