I will answer to the best of my ability and try to keep my opinion to a minimum.
Combat: DA:I continues the trend of having a 4 person party who's members you can switch between. You have the option of playing in an action oriented mode in real time or using the pause and play to issue commands, a new tactical camera has been added for this. The tactical camera lets you choose the abilities or position of each character as well as seeing the enemy health and status. You can now only use up to 8 abilities in combat and you have to set them as hotkeys before a battle (you can't change them during combat). You can no longer use extra abilities through the radial menu. Healing magic is gone from the game with the exception of one "limit break" type ability that consumes something called focus which you build by killing enemies and can use it to preform special abilities. You are no longer able to view or change the tactics of your party, you either select their abilities manually, or they do their own thing. You can enable, disable, or set abilities as preferred, but your companions have an 8 ability limit in combat too so make sure to set those beforehand. Your potions are limited now as well and health potions can be refilled at camps and such while special potions can be made with herbs at a camp or base. You have to set them to your radial menu in order to use them (I think you can only set 3 types at a time). Oh and archers can move around while shooting now (also there are bowstrings!).That's all I can think to write about the combat system for now, but maybe someone else will expand on it.
Other gameplay: DA:I is focused very heavily on open world exploration and you must accrue a certain amount of "power" points to unlock new areas and progress the main story. You can get these by doing sidequests, killing dragons, establishing camps, closing rifts, and so on. You can choose your race out of Dwarf, Qunari, Elf, or Human and each has a certain amount of race specific dialogue. There is also specific dialogue you can choose if you're a mage, or a human noble. No matter what race you choose you can pick between two voice actors for each gender. You can now make your own weapons and armor with materials and schematics you find. Since the world is large and open, you can pick from a few different types of mounts: horses, harts (giant elk things), dacolisks (dragon looking horse creatures) or giant nugs in various colors and patterns. The war table is an important mechanic in the game and has you sending agents to do different tasks in order to get influence, items, unlock areas, etc...You can have a romance in DA:I and have 3 female and 5 male options out of your companions and advisors.
Music: I thought the music during cutscenes and dramatic main plot moments was very beautiful but to me didn't have as distinct of a style as Origins' soundtrack did. The different open world zones don't have regular ambient music, and instead have occasional clips of 10-30 seconds.
Multiplayer is not my thing so I never tried it, but it seems to be fairly popular on the boards so hopefully someone will be able to answer your questions.
I hope some of that might help 