My expectations for Oblivion were fairly low. I liked Morrowind and Skyrim in different ways, but the most important thing in those games for me is exploring the world. Based on the gameplay footage I had seen, the world of Oblivion seemed so uninteresting that I was unenthusiastic about starting this game. Now, after 50 hours of play, of exploring every location and completing the most appealing faction quests, I have to say I'm positively surprised. I'm not all that interested in continuing the game further, but looking back, my time with Oblivion is mostly pleasant.
I would say my assumption about the world being pretty uninteresting is accurate enough. Cyrodiil is a middle-European, medieval mush with few distinct locations. To the north lies some icy mountains, but past that it's the same green forest most of the time. In the center of the map is the Imperial City, which looks cool from a distance, but doesn't have anything interesting to it once you get there. It's the same area copypasted 6 times, which can be said for Vivec City from Morrowind too, but at least Vivec has more distinctive architecture and layout. Thankfully the other cities are constructed differently: Bruma to the north has Scandinavian loghouses similar to those in Skyrim, Anvil to the west has higher-class houses made of stone and brick, and Bravil in the south is an ugly shithole with slums made of wooden planks.
However, where Oblivion does shine are the quests, especially the Daedric shrine quests and guild missions. They feel more varied than anything in Morrowind or Skyrim and feel more, uh, natural? For example, in the Thieves guild, missions often start with looking for and bribing beggars for information, and communicating with other members for more missions is done in secret. Daedric shrines demand gifts before even talking to you and can have some weird requirements, for example being ugly (Personality < 20). Missions can range from killing targets with a stuffed animal's head to causing havoc in a party by casting a spell that strips everyone.
Dungeon crawls do still exist unfortunately, but they're not as excessive as in Skyrim. I steamrolled through the game on the easiest difficulty, so I spent less time in boring dungeons, and I think this might just be the ideal way to play an Elder Scrolls game. In all games the depth of combat is basically "chug potions, spam left click", and increasing the difficulty just increases the time required to kill an enemy. Morrowind had some ridiculous difficulty spikes, like an enemy with 700 luck, whereas Skyrim just had far too much dungeon crawling. By the time I "finished" Oblivion I've spent more than 200 hours playing the three games, I think I can safely say I'm sick of the repetetive combat. I can't really tell which game's combat I dislike the least, but at least in Oblivion it was over quickly.
Maybe the biggest surprise I had while playing Oblivion is how fantastic the soundtrack is. I had only heard the main theme previously, and while the main theme does not compare to Morrowind and Skyrim, the rest of the soundtrack might just be some of my favourite in the three games. It has this incredibly relaxing vibe to it, and it isolated me from the real world the most. That is not to say I don't love the other soundtracks aswell, but Morrowind's soundtrack is far too short and very clearly just loops tracks, Skyrim's soundtrack sounds a bit too ambient at places, and Oblivion gets closest to the sweet spot out of the tree. It's very memorable but it also doesn't repeat every ten minutes.
One of my biggest problems with Oblivion are the technical hiccups, like fairly frequent crashes, which might or might not stem from the anti-stutter mods I had to install. Even with those, framerates could drop to bloody 30 in some places, even indoors. Mind you, this is not a very good-looking game, it just doesn't seem to benefit from more than one core, and I doubt it utilised my graphics card all that well either.
Other big issue is the voice acting. Christ, adding voice acting to your game does not automatically make it better. You're making a massive open-world RPG, why the hell would you feel obliged to shrink the amount of lines characters have just so they can have a voice? A voice that at least a hundred other characters share? How many voice actors does Oblivion have, 5? 7? I can't tell but I'm almost convinced it's less than 10. They try to differentiate it by having the voice actors badly imitate different voices, like the voice of an old person or a beggar, but goddamn, it does not work well at all. The voice acting by itself sounds unconvincing, but sometimes characters even change their voice actor mid-conversation! If they don't change voice actor, then they very often change their tone, and even that sounds incredibly jarring. It's quite remarkable really, the voice acting sounds simultaneously monotone, uninspired and flat, but also psychopathic and inconsistent. Skyrim doesn't do much better here, but what was wrong with Morrowind's system? I can read faster than characters can talk, and by not having voice acting the writers can write more and also change and adjust the dialogue all the way before release. There also aren't completely bonkers tonal shifts, but at the same time they can get more creative with different personalities. I understand people have come to expect voice acting in their video games, but that doesn't mean it's the best decision.
If I had to rank Morrowind, Oblivion and Skyrim, which I basically have to do because who doesn't like arguing about video games on the internet, it goes Morrowind > Skyrim > Oblivion. The gap between Morrowind and Skyrim is bigger than between Skyrim and Oblivion, but I hesitate to call any of these games better than good and worse than decent. For a person who has never played an Elder Scrolls game before, I would recommend Skyrim -> Oblivion -> Morrowind. That way technical hiccups don't ruin your fun early on and you have less to take in at first. For those who have played just one or two of the three games and enjoyed them, I recommend playing the rest too. However, I highly doubt I would like the other Elder Scrolls games, as the main draw for me is exploring the game world, and Daggerfall's randomly generated world seems quite dreadful.