When followers hit level 100 they unlock item lvls much like your hero. If you don’t have much time to play at the start, they can quickly outlevel you in both normal levels and ilvls until you have some time to catch up!
And the Hearthstone upgrade that isn’t really an upgrade but a replacement (it replaces a trait)
Later on with a war mill in operation, you can churn garrison resources into new sets. You can also get 630, 615 armor sets or weapon sets from the 615 quests or war mills.