First, I wanted to stress out the design philosophy. There are no changes for the sake of changes. The game overall feels and plays the same. The changes are not revolutionary but evolutionary. Only features that actually improve the game, without destroying everything for the sake of novelty, were added.
New features in the sequel
The list below contains only features that are already implemented and tested. Those are not all the changes. But what is listed is guaranteed to be in the game.
Item suffixes
The whole items system was redesigned. Now items can have suffixes (like "Shield {of Fire Resistance}") which greatly increases the diversity of items. In addition there are 3 quality tiers for items (so "Shield {of Fire Resistance} [II]" is better than "Shield {of Fire Resistance} ", even though those share the same set of abilities). Also, the random treasures generator was rebalanced to give more equal ratio of various items. And the last thing, because people will ask, yes, there will be magic staves to be found.
Shops
There are 4 types of shops now. Regular shops which provide trivial items. Guild shops which provide random set of items, Pawn shop where you can sell and buy back unneeded items and Magic shop where they accept crystals only. Shops have separate stock on a per town basis and further towns have higher tier shops (better and more expensive items).
Resistances system
Now resistances are not 0/1 but are value based. So, as the monsters become tougher you need to obtain higher resistance values (with extra options for temporary boosts). Next, Acid was replaced with Sorcery which makes it more thematic and some resistances were merged (for a total of 8 resistances).
Towns as separate locations
Widely requested yet quite a simple thing, towns are now not as a menu but as full fledged maps you can explore.
Skills
There are party skills and trainers scatted around the world which teach such skills. Those provide nice bonuses and an excuse to explore the world even further.
New dungeon features
Extra features in dungeons like illusionary walls, doors locked by keys, etc.
More complex connections between locations
It might be not instantly visible to you as a player, but it's very important for designing maps for me. Now there are two way exits from locations and inter locations portals. This allows me as a designer to make interesting topography like you enter a cave in one place and exit it in a completely different place on the overworld or you enter a portal in a tower and exit inside a dungeon on the other side of the map deep below ground.
Fountains
Wells function the same as in the predecessor (healing), but fountains were redesigned and now grant temporary bonuses (resistances, attributes, etc).
Griffin travel rules change
There are subtle, yet important changes to the way travel via griffins work. For example, now griffin travel takes time, which effectively means you can not use it to go and visit every single spot with free buffs (because first temporary buffs will expire after a several griffin runs). Also, griffins can no longer land on lava or desert, making traversing dangerous terrain much more tricky.
Resting rules adjustment
You can no longer rest on lava, which combined with new griffin rules means that lava terrain becomes a real challenge. Oh yes, also now you can rest in inn without using food by paying the fee directly (which was requested like by everyone).
Map shows a tiny minimap with overworld
Now you can see the whole shape of the overworld at a glance by looking at a tiny map which was added on the full map. Very handy, you will love. Trust me.
Field of view extended (unfogging minimap)
Another small, yet highly requested feature. Now you don't need to physically enter every single tile, surrounding will auto mark as "seen". What's best, the information what you visited physically is not lost since a tile is displayed differently if you visited it or merely saw it.
Magical barriers
Those were rebalanced and divided into three grades (with distinct graphics so you can clearly see the danger level of each barrier) now posing a much greater threat. Basically, those can wipe out an unprepared party and are used in locations to slow down your progress or even make you turn back. Lightning resistance is of high value now since it can greatly reduce the magical barriers damage. Overall, now magical barriers are a serious threat which will make you reconsider how you explore dungeons.
Small adjustments that go well together
There are other small changes which bring synergy, for example drinking from fountains takes one hour which combined with the added time for griffin travel and the fact that temporary buffs expire at midnight let you use griffin to visit 2-3 fountains before an important fight but not to visit any number of those. At the same time drinking from wells (healing) takes mere minutes so it can be used as an effective local healing in a series of fights without the fear of expiring party bonuses.
Many small improvements
There are many other small things not mentioned here. Additional art assets, animations for portals, etc.
Possible other features
Since only things that are already implemented and tested were listed above it means there are decent odds those are not all the changes that will end up in the final game.