Can someone list the primary additions this expansion adds? Specifically I'm wanting to know if you can now use magic while mounted, and if there is indeed the ability to respec a character? Thanks.

Too much to list, yes you can respec once the main quest is completed. You can cast magic while mounted on a dragon, dont know for sure if they added it to horse mounts, dont think they did.

You can respec the perk points (after the dragonborn main questline, at the cost of one dragonsoul for each perk tree you wish to reset), but not your health/magicka/stamina.

You can indeed use magic while mounted, atleast while mounted on a dragon. I havent tried if they added it for horses.

The dlc adds a new island with both a main questline and some side quests/questlines.
New enemies.
New alchemy ingredients.
New potion effects.
New armor types.
New weapon types.
New enchantments.
New magic.
New powers.
Dragon riding (not free roaming, but still).
New followers.
New pets.

Hmm... did i miss something?

I heard the Altar of Thrond makes an appearance here. Does it do anything?

Appreciate the reply and info.