In Morrowind you can soul trap gods and fly through the air around the mushroom tower you grew yourself or through tunnels thousands of kilometers below ground. The atmosphere and the setting is incredible, the Dunmer used to be Daedra worshippers but now worship mer-gods, corrupted beings still clinging to former glory, there's ash storms, a giant volcano surrounded by a great magical wall, Blight storms spreading the gruesome Blight disease, a truly and surprisingly amazing story, different and otherworldly and I didn't experience it before just a few years ago so it's not nostalgia talking, great characters (like the first blades member you meet who is a naked acrobat hand-to-hand fighter skooma addict), a slightly steam-punkish universe, the sheer brutality and violence of Vvardenfell culture, for example most Mage Guild quests are mere internal power struggles where you basically act as an assassin (if you care to) and this dark, unforgiving theme just hangs in the air all throughout all of the quest lines. The universe is something as rare as an original fantasy epic.
There's an almost limitless crafting and spell creation system (where in later Elder Scrolls values are in the hundreds and sometimes a few thousands in Morrowind we are in the hundreds of thousands), I've never felt so powerful in the late game by a great margin, and I've rarely felt so free to explore different character builds. The lore, the openness and dynamics of the quest system, the amount of different professions you can have, join the Imperial Cult, join House Redoran, join the Telvanni, join House Hlaalu, the Thieves Guild, the Dark Brotherhood, the Temple, the Legion, the Ashlanders, the Blades, the Fighters or Mages Guilds etc. and mostly there's not just one place to start or even continue these faction story arcs, you'll get a different experience and entirely different quests by talking to different npcs or by visiting different cities or regions.
In Morrowind you can read and listen to the lore and think you know how history has developed before your time then by a rare chance, find that extremely well-hidden room that contains a secret library and discover illegal texts kept under guard by the Temple learning that everything you thought you knew is a lie, and that the outcast Ashlander nomad wise women actually are in the right, and you could just as easily never have found that room in that playthrough at all.
The amount of skills and attributes is simply staggering (you can use weapons like spears and halberds!), you can level up in almost anything. Morrowind is also the only game where you can, like I did in my last game, just become a craftsman, I joined the Mages Guild as a journeyman and started doing alchemy and trading, until I become very, very wealthy, the money of which I spend on training my skills at the numerous trainers (no skill training limit per level at all with enough money you can level yourself infinitely) and I travelled from city to city trading, doing my craft and training under various masters and didn't go adventuring until level 40, ditching weapons and armor entirely except for a noble outfit and a ceremonial spear.
In Morrowind there's also no quest markers like in a real rpg and you have to use your brain and your own two eyes and journal and talk to NPCs whose help are not always infallible, you explore in a completely different sense than in modern rpgs. There's also a staggering amount of dialogue. Yes, much of it is reused on different npcs but if you are a careful and dilligent reader you'll soon learn how much there really is, and how snippets of information can turn your whole world on its head.
Oblivion on the contrary was a great game, I played it for years, but due to the item and creatures levelling system unplayable without mods. It also catered way too much to casual gamers and children (the main devs have even explained that they all had little kids when the created it), the dialogue is simply embarrasingly bad and oversimplified, and so is much of the story and many of the quests, silly, infantile and pretty stupid, and something just feels off about the atmosphere, too light and too bright, easy calories and too little depth, despite some good parts. Luckily Bethesda returned more to its old Morrowind style in Skyrim, which if you only played it for weeks you should consider taking up again. At first I was sorely disappointed and I even gave it up for a good while until I forced myself to play it again and I've played it for thousands and thousands of hours since on many different characters. Go explore a bit, try the expansions, especially Dragonborn, the main quest line is the weakest in the game.