Here’s my take… it’s gonna be a long post, but I’ll try to stay on topic 
I like the new rating system in that it lets you have more meaningful progression break points for ilvl upgrades. This works much, much better than the old way of timing all keys at X level because sometimes it was really difficult just to find the one run you needed. Now you just need the score and there’s a lot of flexibility in how you approach that.
Also, while I may have reservations on rating itself and its impact on the community, with it being baked into WoW means all players have access to it. Very quickly players will come to a generally understand what it means and how its used, with no need to use 3rd party addons or a 3rd party website. However, I do like that Raider.IO is complementary now. The built in systems tell you your rating, but RIO gives you access to your history. The two complement each other really well!
Here’s the downsides… M+ has the potential to be its own progression system and I think a lot of people want that. Unfortunately, those rating breakpoints for ilvl upgrades are really easy to get. Going into 9.1 with roughly 226 ilvl from last season, mostly from box upgrades (as I’m a Heroic raider), I’m fully capable of getting 2k rating right now with a group of people who know what they’re doing. I’m at 1850 (ish) rating and I’m just waiting for groups and keys in my guild to finish off that last bit to 2k.
So now I feel like, at the very start of the season, I’ve already progressed through most of those gear upgrade breakpoints. I have my 2k achievement to go but it feels like I got through the others too quickly. Contrary to this though, for any kind of meaningful gear progression, I’m still valor blocked and will be for quite some time. There’s a disconnect between the progression gates to upgrade the gear and the rate at which you can upgrade your gear.
There’s an opportunity for this to be better. I like the idea of working through the M+ ranks to improve my character, and I like the idea of gearing up to do harder and harder M+ keys. Gear should be meaningful in this progression and I think there needs to be some tweaks to how you advance through this, and the rate at which you can upgrade your gear. I should feel meaningfully rewarded for each dungeon run I do, but I don’t need to jump up in big power spikes every time.
Then there’s the core issue of Tyrannical being required for 2k rating. I mean, it’s not really required, see my above points about flexibility in the system and that being a good thing, but the root problem is that people weren’t enjoying Tyrannical weeks for a reason. That reason varies from person to person and I think it’s really easy to be critical and condescending of people, but the bottom line is that with so many people not enjoying something the solution isn’t to just try to force people to play it.
Some attention needs to be given to why people don’t like tyrannical. Maybe more tweaks need to be made but I think at this point, maybe the whole affix just needs to be evolved into something different. I can only speak to myself, but Tyrannical weeks tend to just make dungeon runs less interesting. Trash taking longer doesn’t feel as bad because it’s not that much longer per pull, and trash packs are part of the M+ dungeon puzzle experience, in that you can optimize the pulls, or tackle them differently per affix. Bosses, on the other hand, tend not to change with the affixes making them just take longer feels dull. Mechanics in WoW are my favourite thing, and I think no other MMO comes close to WoW on this front, but that doesn’t mean that it’s fun to go into a repetitive sequence of boss mechanics just because it’s gotten more HP. Taking a good thing and just making it longer doesn’t mean it’s going to become better.
So anyway, that’s my views on the new M+ system. There are some positives, but I think it needs work… work that we probably aren’t going to see for quite some time, unfortunately.