Solstheim is small and can be easily recreated with Skyrim's existing resources. There will be 5 new dragon shouts and 15 barrows (using the same names but redesigned), as well as caves and other existing landmarks.
Vvardenfell, due to the eruption of Red Mountain, is quite barren. You will encounter a few caves and ebony mines as well as the ruins of familiar locations. There will not be much content here or much to see, but Morrowind players might shed some manly tears.
The rest of Morrowind includes the west and east. The west features Blacklight, the east features Port Telvannis. Both include dwemer ruins, ancestral tombs, caves and settlements.
Now for the quests.
The main quest picks up in Solstheim after Alduin's defeat. You are called by either the Blades or Paarthurnax to deal with dragons there. You take a boat from Dawnstar and arrive in Fort Frostmoth.
Events lead you to a falmer-infested cave that opens up into a large dwemer ruin. The rest of the main quest involves the disappearence of the dwarves.
You obtain Sunder and Wraithguard and from the Red Mountain crater and restore both them and Keening to their original power.
Divayth Fyr has relocated to a tower on the Telvanni Isles, and you can guess who is with him.
This main quest is just about 3/4 as long as that of the base game. That's all I'll spoil for you.
As for the rest, each faction from the base game has 5-6 quests added on in this expansion.
The Companions have moved into Thirsk and are attempting to bring the werewolves of Solstheim into the fold.
The College of Winterhold has some problems with House Telvanni.
The Thieves Guild is at war with the Camonna Tong in Blacklight.
The Dark Brotherhood is establishing a new sanctuary lead by a surviving member of the Morag Tong.
There are also some additional side quests and miscellaneous objectives.
Armor includes chitin and netch leather for light, ice and bonemold for heavy. Weapons include chitin and ice. New robes and clothes. There's also some unique weapons and armor.