I just listed a very specific open world content being added beside dragonriding. The elemental invasions. It even has unique high quality transmog rewards for every armor type.
The reputations being improved with the Renown system as well, with added benefits of making it easier for alts to get the later profession related rewards, which mostly fixes the issue some players have had with profession progression gated behind reputations.
Zones are significantly larger, which changes the feel of content being consumed in said zones.
And of course professions are being given a lift in terms of importance, which ties more into the open world gameplay. The fact that the professions give armor that is higher quality in terms of ilvl and appearance also means that said open world gameplay will have a more distinct progression.