If i was to list all reasons, i would have to elaborate quite significantly, and thus the next ten posts would be filled with my rambling. Let's narrow it down to a short :
Repetitiveness, Level Scaling, Ridiculous plot holes, Two voice actors total, The same god damn animations for every NPC, Persuasion mini games, Dumbed down abilities, The fact, that EVERYTHING was in the colors of rainbows, and was shining like balls. "Oh my god! It's a bloodthirsty daedra.. shining, bloodthirsty daedra, on a glade full of shining flowers and sparkling rocks.."
Level Scaling should be mentioned at least ten times, and it still wouldn't describe how BAD that idea was. After you reach level XX, every single unwashed, dirt eating, outside sleeping bandit will be equipped with the most powerful armor set in the game. Am i exaggerating? Try fighting the unwashed arena champion on the hardest difficulty. After you hit him 500-800 times then perhaps his health will start going down.
The guards in each city are always higher level than you (YOU stop the god damn deadra, guards!)
..And having 10 "Close Generic Oblivion gate" quests in a row.. No.
Not to mention the absolutely broken, overpowered gear of 100% damage reflection, 100% chameleon, 100% absorb magicka.. How fun! NOT.
And Many, many more things.
For me Oblivion was one of the most disastrous mistakes in the last decade.
Aside from Graphics, combat system, lifting objects and dark brotherhood quest, it was worse than TES3 in every single possible way.
You, viewer, are entitled to your own opinion, and if you disagree with me - I'm fine with that. But no arguments are going to change my mind. Please, let's not talk about TES4. Thank you.
Oi! (back to installing mods!)