Patch 9.0 had an interesting gear optimization metagame, where arguably the easiest way to gear up was PvP through the easy-to-use Conquest vendor, though Raiders and Mythic+ players also earned very powerful gear from the Great Vault system. After a few months of the patch, players could be within a few percentage points of the most geared players even without engaging with every form of content or even any specific one, though 233 items eluded all but the most dedicated. 9.1 shifts this paradigm with nerfs to PvP gear such that they'll be more quickly replaced in PvE content, though still very strong and worthwhile for top-end players at the start of the patch.
Patch 9.1 also introduces Shards of Domination
: powerful gems that only go in sockets in 5 specific gear slots based on your Armor Type. Notably, the gems and the Domination Socketed items only drop from the Sanctum of Domination raid, and most of their power functions outside of the raid (the "set bonus" effects only work within the Maw and the raid, however). Some of these effects will be very noticeable in dungeons, such as Portentous Shard of Cor
, so the difference between a player who has access to Mythic Raid gear and a player who does not will likely be bigger this patch than it was last patch, where eventually you could catch up in item level through the Great Vault alone.