Altars of Lilith - Renown will be reset on a seasonal basis, however the stat gains will not be reset.
The skill trees at launch are only the beginning - new skills and passives will likely be added in the future, but these are likely Expansion changes, not Seasonal changes.
The workaround for killing a world boss multiple times in rapid succession has been removed.
Skills have been normalized based off Aspect Ratio for monitors.
The Reset Dungeon Button was added in early development before creation got to the point of allowing for other methods to reset, and had unintended consequences - while the button may be removed, dungeons can still be reset fairly easily.
Den of Evil mechanic will now have monsters chase you down instead of having to search for one missing monster.
Objectives have been redesigned in some situations in order to minimize poorly received layouts - you'll naturally encounter objectives along the way instead of going back and forth.
Numerous dungeon changes - in particular regarding backtracking.
Every class is designed with a good set of options to clear Crowd Control - any issues at lower levels are mitigated at higher levels.
Classes alone weren't tweaked - Bosses in particular were reevaluated for Melee and Pet Build viability.
Druid and Barbarian feedback was especially helpful.
Numerous class changes - documented here in a previous news post.