It wasn't bad to everyone. And it is common for the game to be compared to its predecessors. Some of the criticism has been valid but much of it isn't. I think that Oblivion was really innovative in a few key areas but some people coming from Morrowind felt that it didn't allay itself as being a spiritual sequel to past Elder Scrolls. In the end any reasonable person will acknowledge Oblivion for its accomplishments and see it as a well made game, even if it didn't have all of the same elements as Morrowind.
In short, both games are well made and they both present a different flavor of game play. When some players who tasted the flavor of Morrowind (or Daggerfall or Arena) and then tasted Oblivion they cried "it's not the same!". But most persons should acknowledge that each flavor tastes good.
That's all I got this morning, my brain still waking up
EDIT
Also, my own personal criticism (I know it has been stated by others, but just to add my voice) when Oblivion came out I didn't like the level scaling. So I used a mod (Franscesco) way back when to help keep monsters at a static level. But I will say that I played vanilla first, and modded it later. Most of my game play after several hours in vanilla was with very few changes. Just Frans and cosmetic texture changes.
Later, many years later (2010) I went back and played Oblivion with about 150 mods. It was a lot of fun, and some don't feel it is valid to rate a game based on the mods but it is a valid criticism because, regardless of where the mod comes from or who made it, they are available and increased the longevity of the game. Oblivion has an absolutely incredible modding community, and that modding community has given Oblivion a special place in history as one of most versatile and personalized games ever created. To each his own, with mods