I didn't hate Oblivion, I just thought it was just a big, steaming puddle of generic meh. Reasons for not liking it include:
Lockpicking minigame for allowing a character with a lockpick skill of 10 to pick the hardest lock in the game.
Speech minigame for turning persuasion into a gimme, eliminating the need for developing the speech skill.
The AI for eliminating stealth - bit of a weird one but try this: create a spell that has two effects, invisibility on self and say fire damage on target. Casting this spell will keep the player invisible at all times yet monsters will always run to the very spot you are and stand there, because the AI subroutines immediately allow creatures to know the exact location they took damage from. This eliminated stealth as a viable way to play the game.
The stripping away of a large number of skills from Morrowind. To some people this is a minor thing, but the loss of skills such as Unarmoured made character builds like the Monk all but useless.
The lack of additional lore in Oblivion. The Elder Scrolls games have always had a rich base of lore, and what many players new to the series don't realise is that pretty much half of the books in Oblivion were copy-pasted from previous Elder Scrolls games.
The replacing of the unique creatures of Tamriel with the same old Minotaurs, Ogres, and wolves that appear in every single hack-and-slash since the dawn of time ever.
The removal of faction competition. It was possible to have one character elevated to the head of every guild and organisation in Cyrodiil, meaning that characters no longer had to weigh the consequences of their allegiances and actions.
The endlessly copy-pasted Oblivion Gate levels which made completing the main quest one long, boring grind.
The horribly limited voice-acting pool which when combined with the 1000 yard uncanny valley stare made NPC interaction distinctly unnerving.
Y'know what, I could pretty much go on all day here, so let's just take it as read that I thought Oblivion was the weakest TES title to date and leave it at that. Skyrim has done a lot to get the TES series back in line with the tone and style of I, II and III, and that alone is reason to praise it.
Plus it brought back werewolves.