Using "Search" would probably be best - there's about 98837635 topics asking the same question.
Anyway, for me it's the difference between Morrowind and Oblivion and all things that were lost. People who started with Oblivion usually love it - you're a good example. But those who played the earlier games tend to complain a lot about the changes they didn't like.
The colors. Awfully bright colors that hurt my eyes.
The faces. They all look the same, they don't resemble real faces at all and are just ugly.
The "generic medieval fantasy" feel. Some people like it, some don't. I admit I enjoyed it, but those who were used to Morrowind's weirdness expected something different. Some wanted to see a jungle.
Butchered lore. Say what you want, the main quest is just ridiculous.
Where's my levitation?
Quest markers. Compass. Fast travel. It all started there.
There's a lot more, but I won't list it, because I quite liked Oblivion - even though I never managed to finish it