I misread it and thought at first that M+2 to M+10 wouldnt have any timer, but that is not the case.
It seamse instead that they beefed up both Heroic and M0 tro replace most of those stages and M+ then starts at a higher iLvL.
Still a nice improvement for casuals but still keep the competition for M+.
Yeah, essentially its all squished to lower levels.
A full set of 5s will give you the rating equal to 15s for the mount reward as it looks right now.
changing it all completely. And 12s / 14s being average ‘‘keys’’ atm will be high-end keys in Season 4 and The War Within.
Because the 2-9 difficulties are removed. Well done.
Flightstone earnings for any given Mythic+ will match the equally challenging Mythic+ from previous seasons. This means that a Mythic +2 in Season 4 will give the same number of Flightstones as a Mythic +12 in Season 3 currently does. The rewards will continue to match difficulty, even if what we’re calling those difficulty changes.
How odd, the blue post specifically gives an example of how much Flightstones a M+2 in season 4 will reward you with.
Ow and there is a whole table with info about what loot is granted from the great vault which Punyelf copied in here at the top.
Somehow M+2 has a weekly reward in the Vault, how weird if i cant run that content 
Apparently for not being able to run a M+2 dungeon, i can get a itemlevel 509 item in my weekly vault.
That is a weird example, what use is that info for me if i cant run M+2 dungeons?
Wouldnt the example be way better with giving me a +10 mythic run and the amount of flightstones rewarded? And the appropiate itemlevel i get from the weekly vault?
M+ 10 itemlevel weekly vault: 522 itemlevel, the new maximum itemlevel in 10.3.x
Once again: key levels are squished, +20 will be the new ‘‘maximum key’’ possible (like M+31 or M+32 currently is)
since they are completely changing that.
You just miss the comprehension that i say the current difficulties of +2-9 are removed because M0 will be M+10 and heroic M0 without mechanics in the new season. And you try to attack and get personal over that that i need to learn to read.
Those difficulties arent removed, and some had overlap to begin with in terms of rewards.
And each new M+ level will be more rewarding and feel useful.
if you can run +7s, there is no need to go and ‘‘collect’’ a full set of +8s and progress your way up.
the new system totally removes that.
Making it way better and makes more sense, in terms of scaling and item rewards. nobody can really tell the difference between a +7 or +9 in terms of scaling.
the new system will, because each key level is more pronounced in the scaling / difficulty etc etc.
M+2 - M+9 will be M+2 through M+4 basically.
So where are they? Heroic is M0 without mythic mechanics and M0 will be M+10. Where are +2-9? You tell me.
They were bloat keys to begin with. So if you were ‘‘able’’ to clear all +5s in Season 3,
Season 4 will ‘‘limit’’ your keys to play to M+2 / M+3 tops.
In other seasons you could pick out of 4 different ‘‘key levels’’ with almost no variation in difficulty. if you could run a +5 in terms of dps / mechanics, a +6 wouldnt kill you nor would you lack the dps.
The new season just shrinks it all down into less key levels, and be more meaningful during the whole way.
So, nothing is removed, it is just named differently.
If you can only clear M+5 currently, in the new season you are only playing M+2, but with similar rewards as if you were playing M+11 through M+14ish in Season 3.
And that was maybe for you but not for me. I definitely feel the difference in +2 or +4, or your previous example of +7-9. But the current system is completely removing 2-9. I have chars i never get out of the +7 range.
It is great for you you can instant jump in +10 but i can’t.
Well, in terms of scaling the differences between the keys is very small.
And changing the names and difficulty + scaling in season 4 might still mean your ‘‘max M+7 key chars’’ can be played by you in M+2.
but that might be the new ceiling then.
since its all condensed into less numbers / key levels.
So you also contributed to distorted statistics 
I would have been fine when they kept +2 as +2, but make the keylevels with a bit more difficult per keylevel. even like 2 keylevels, so +3 would be current +4, +4 would be +6, etc. But now they just removed the learning ground to get into it.
It isn’t distorted when i am not good enough to get out of those levels. But for your information, i add 75% instead of 50% so i make the percentage without 10’s go down instead of up.
But that happens regardless, wether or not they retuned dungeons so that M+2 till M+10 is the new M+2 till M+20 in the same 10 levels.
Every season, keys got scaled up and dungeons became more difficult in terms of raw numbers.
So your current ‘‘M+7 char’’ is actually playing a M+18 key of Season 1, in terms of raw numbers and ofc relatively to the powerlevel we had back then.
So with Season 4 coming with new naming for dungeons and scaling, there are a few options:
i play multiple classes each season in M+ on varying levels and when i hit my ‘‘ceiling’’ of playing X class than i have to get better.
if i dont want to do that: so be it, thats it for me.
The thing is that the M+ system we have had so far was extremely bloated with a lot of useless steps.
I mean if you do a pug today, why join a 10 when you can join an 11?
Both are close to the same difficulty but as 10 gives you drakes, the 11 gives you Wyrm.
Thats the case with M+ in its current state.
The ones you do is m+2, m+6, m+11 and m+16. To do the ones in between is just a waste of time unless you need to uppgrade your key.
I do think this new system will work great.
A higher threshhold before going to M+ and fewer steps in the M+ ladder.
Heroic and M0 is practically redudnant today, so they made them important again.
They are doing something else this season. They remove +2-9 difficulties. They never did that. What they are doing is what normally would be, in every other season, a M0 will be scaled to +10. That is new. That is why i say my nice slow learning ground is removed. I am fine when everyone else is just able to jump in +10, but i am not.
But you do have Heroics that have been upscaled as well.
To M0 without mythic mechanics. Don’t you see the big gap between M0 without mechanics to M+10? There’s nothing in between. The nice slow learning ground is gone.
M0 is with mechanics, without affixes.
than M+2 is the first M+ key with affixes which is either Fortified or Tyrannical depending on the week.
I would wager a guess and say that if you can do +7s / +8s you arent completely ‘‘dead’’ in a M+2 in season 4.
and you will be fine.
but maybe M+2 is the only difficulty you can run, in before you had multiple keylevels that werent a big enough step to give a way bigger challenge.
I do think you are making a bigger deal of the steps you did in the past between +2 up to +7s, since the steps never were so big, especially not since there are no affixes coming in either till a 7 key currently (the 2nd affix)
Overall the removal of all the ‘‘redundant’’ levels is great to me. like Teezah said, we have too many keys that introduce bloat. And we have certain ‘‘breakpoints’’ later in the season showing only certain key levels instead.
and finally: weekly vault from filling it with M0 runs: a ilvl 506 reward, which is fairly high if the cap on items is up to 522 ish
M0 Do have Mythic mechanics albeit no affixes.
What they have done is to prolong until when affixes arrive but still have the difficulty befor they do.