SNES To DS Changes

You now control Cecil in battle in the opening sequence
Auto Battle System
Cecil cannot equip bows or staves
Rydia, Palom & Porom cannot equip bows
Rosa can use daggers
Arrows are not consumed when shooting a bow, and come in quantity of 1 from chests
Characters rejoin at set levels
Tellah is now able to earn sufficient MP above 90, with sufficient level ups. (thus allowing meteor to be cast early.)
Bosses are generally harder and have new attack patterns
Switching weapons mid-battle takes up a turn
Bows are fixed on the right hand, while arrows are fixed on the left
Bows no longer cause extra damage to flying targets
Spears no longer cause extra damage to flying targets
Edge can no longer equip claws
Bahamut's megaflare attack cannot be reflected.

Cecil's dark attack requires using dark then using the attack command
Cecil gains more white magic spells (Protect, Shell, Raise, Teleport)
New magic spells Ultima, Faith, Brave, Bubble (and enemy-only Reverse)
Gilbert can select songs to perform and has a number of new songs
Child Rydia learns different spells over the long run
Bio damage reduced
Can use spells from the magic menu without going back to the main menu after each turn
Exit/Warp is disabled in all Castles and towns. (prevents bugs, like fetching the dark crystal early.)
Warp is completely disabled in the Sealed cave. you now have to walk all the way back out.
Warp now puts you at the start point of the previous room.
Rydia does not learn meteor untill level 80
Osmose consumes 5MP

New flashback scene of Cecil/Kain dueling in Baron Castle
Additional dialogue between Cecil/Rosa in Baron Castle bedroom (?)
Extended scene in village of Mist between Cecil/Kain/Rydia
New scene featuring Cecil's parents and him being found by the King of Baron (playing as young Golbez.)

Rydia has a new companion, the Pochika
Decant Ability System
Auto-map, rewards for 100% exploration per level
New Hidden bosses
Subtle map changes to towns and dungeons (ie: On the peak of Mt.Ordeals, instead of a small shrine, there is now a stone tablet which teleports you to a shrine)
Namingway has a new job as Mappingway (changing names appears to be unavailable due to voice acting)
Kain and Palom are now right handed
Item capacity increase
Various characters have different stat gains
Stat changes past level 71 are no longer random but controlled by equipped decants
Spell damage has increased for most spells
Every character except Cid naturally gains MP
Elixirs can now be bought (from hummingways).
Dr. Lugae after he turns into the skeleton, uses new abilitie Reverse(Causing reverse of damage, healing=damage damage=healing)
You can no longer bring the hovercraft underground.
Avenger sword can be used together with shield.
Poison axe can be used together with shield.

Final Fantasy IV DS