One of my biggest issues is that Cyrodiil was portrayed as a generic Western European fantasy. They could have cherry-picked from real history to spice it up if they wanted to go that route, but instead they streamlined the culture and setting into a bland mush. The whole setting, chivalry, religion, society, etc. was like a B-movie production where the writers didn't bother to do real research and just merged a bunch of half-remembered ideas they'd heard about Medieval Europe. Combine that setting with simplified black hat/white hat story and quests, level-scaling, the compass, and many of the other design decisions and it's a recipe for disappointment to returning fans. It's also natural to want to lump it together into a grand dumbing down scheme--like an Asian or Indian restaurant that Americanizes their food because they're afraid of scaring away timid customers (and thus losing the interest of the foodies who'd be their biggest fans).
I don't actually think that's what they were trying to do, but I do think that's how a lot of fans felt. There is an over-developed sense of entitlement on the forums, and I think Oblivion was a better game than some give it credit for, but BGS titles have high expectations, and they've earned a lot of trust from players, so it's not that surprising for people to feel and express disappointment when BGS switches direction.
Pretty much my thoughts exactly. Oblivion wasn't really a bad game. I did feel it necessary to download numerous mods to fix my issues with it while I played Morrowind the first couple of times without anything like that and loved every second. Oblivion was a disappointment and I really hope Beth learned from their mistakes. It takes great effort to create a gem with the kind of depth Morrowind had.