Hey guys, your friendly neighborhood Blood Death Knight here! I thought I would make a quick post about my thoughts on Mythic+ Dungeons. Disclaimer: This is all from a tank perspective. I have not played a DPS or healing role in any of my runs, so any time I touch on one of those roles, take it with a grain of salt. Warning: Wall of text in 3... 2... 1...
First off, always pick one person to make call outs. This includes using Heroism/etc, stuns, combat res, and when to wipe and reset the mobs. I tend to do this for my runs, but if someone else makes a suggestion, take it under consideration and don't just blow it off; they may know something from experience, or have seen something you missed. Make sure this person is aware of any stuns and CCs you have in your arsenal. Yes, I did in fact use the “CC” phrase. There are certain mob packs you may have to CC if you are struggling with it (especially with certain affixes).
Know your class and spec as best as you possibly can. If someone gives you advice on what to do, don't take it as a personal attack. They want to succeed as much as you do. However: this does NOT mean you should walk around telling people how to play their class. Everyone is going to have their own way of doing things that works well for them. As a group member, you should do your best to improve your DPS/healing/survivability. I understand it can be more enjoyable to play a certain spec, or you may have one specific talent that you always use when soloing. I get that, but for progression (and I do consider M+ progression at higher tiers), if you are not doing everything you can to maximize your performance, you are hindering your group. (I'm not telling you how to play your class. Read forums, see how other people are making talents/specs work well, and try that out. A lot of people told me Breath of Sindragosa was a poor choice during WoD, but that did not stop me from trying to make it work.)
Read up on strategies and guides for all the current dungeons, and run them on normal Mythic to see boss mechanics. It is absolutely critical to understanding layout and bosses in order to beat the timer, and to potentially get extra chests. I will say that if you have 4 exceptional players, you can carry an under geared or inexperienced fifth player in the lower level +'s.
For tanks (and even DPS), don't be afraid to use CDs on large trash packs, and know what externals are available to you. For bosses, I've found DBM is not entirely accurate on when you need to use active mitigation abilities. Sometimes it yells at you, sometimes not. My best suggestion is to just run enough so you know by instinct when you're about to take a ton of damage.
For everyone: Like I said earlier, Mythic+ is going to be progression for some people and group compositions. Enjoy yourself, but be serious about doing it. If you're getting stressed out during a run, mention it to your party mates. They might have suggestions for you, or you might just need to take a quick minute break away to stretch you legs. The most important thing (at least for me) is to have fun, but not at the expense of others. You're all committing time to this, and you're all in it together.
That all aside, here's some numbers and other misc thoughts (again, all my own exp.):
For Mythic+2, DPS should pull upward of ~170K on a boss. Emphasis on should, because depending on how your group can pull trash packs, you might have more time to spend on a boss. Overall group DPS should be around 575-600k for the entire instance. Bosses, trash, deaths, everything. For higher levels, single target DPS should increase by roughly 1/3 of the Health modifier on the keystone. As an example, M+3 increases HP by 17%, so you should be doing 5-6% more DPS than ~170k. This is without the Heroism buff. (Note: This will change pending on the dungeon.)
That's about all I have to say, hope this helps people in some way. If you have any questions or disagreements with something above, feel free to message me. If you haven't read wowhead's guide, you can find that here: http://www.wowhead.com/guides/mythic-keystones-and-dungeons. They go over a lot of the special traits on keystones, and is overall just a good resource.