Morrowind is one of my favorite games ever, here is a huge mess of random features:
way more side quests than oblivion, although oblivion's were more unique(less plain fetching and escorting).
less balanced. You can't hit anything in the beginning due to hit chance and weapon skill. spellcasts will fail too. in the endgame you are god and only the bosses stand a chance(unless you adjust difficulty), everything is pre-leveled, oblivion levels with you
you pick 5 major skills/5 minor skills, the rest are misc. only major and minor skills can level you up, where you upgrade attributes based on the skills you used to level.
way more spell effects. things like teleporting, levitating, jumping higher. Theres also a bug to make spells permanent for fun.
more in depth spellmaking/enchanting than oblivion. You can make a spell/enchant do several things at once, you can hire a spellmaker/enchanter at any mages guild.
enchanting is its own skill. it has a chance to fail unless you hire a mage. equipment can be enchanted with on hit/on use effects. These allow a non mage to use spells with no magicka or skill. however you must know a spell with the same effect to be able to enchant that effect. they recharge both over time and with soul gems.
Environment is very foreign. The entire game except for the expansions takes place on a giant volcanic island called Vaardenfell. Morrowind is the mainland continent, where you can only visit for the Tribunal expansion(the capital, Mournhold). It is more dark and grim than oblivion's vibrant landscapes. lots of valleys, mountains, rocky/volcanic areas, swamps, coasts, marshlands and weird flora.
the cities/towns are huge and are all open, except for Mournhold in the tribunal expansion and parts of Vivec/Molag Mar. Think of Vivec and Molag Mar as giant citadels built over water, where the city is inside. The towns mostly vary based on the great house they are controlled by. Some are cleanly cut rock, some are giant crabshells/bones, and some are giant organic fungal growths.
IMO main story is much more interesting in morrowind than any TES after it(I have not played arena or daggerfall). it involves a lot of background lore, religion and politics. The overall lore of morrowind has a huge focus on the clashing of religion and major factions.
The text based conversations add a lot to the lore because it avoids the time/cost limitations of voice acting, so the npc's can tell you a lot more. You can ask any npc about multiple topics, although most of the npcs kind of feel less unique because a lot of topics are just repeated.
there are a TON of factions to join, which adds to the reason why there are so many side quests. However, the factions don't really have their own central storylines like they do in oblivion.
No map markers for quests, everything is written and updated in your journal. This is good because it encourages exploration, and has more immersion, and bad when the descriptions are unclear/when your deep into the game, and your journal is a goddamn textbook AKA very difficult to navigate.
Transportation: The only public transports are boats/siltstriders and mages guild teleporters. boats can only go to other port towns, silt striders are giant flea taxis that can only go nearby towns, teleporters go to other mages guilds. Then there are spells: mark/recall, almisvi/divine intervention to teleport
exploration is encouraged because of the lack of fast travel, there are lots of dungeons and tombs with good loot, sometimes good artifacts(unique equipment).
I'd say both expansions are worth it, bloodmoon adds more content, and a whole new island with werewolves and a wintery/skyrim environment (solstheim, a lot changed between skyrim/morrowind times). Tribunal is much smaller, it just adds a city and a big sewer/cave network, however it continues the main story.