Morrowind had more guilds to join than Oblivion, but few guilds had good quests save some Imperial cult & Great house Telvanni quests (and maybe Morag Tong too). Oblivion's entire Dark Brotherhood & Thieves guild quest line beats pretty much every quest line in Morrowind, imho.
The thing that irks me the most about Oblivion is that there are hardly any books around, unlike Morrowind. Collecting books was one of my favourite parts of the game. Also, they could've kept at least Morrowind's mage teleportation and divine intervention into the game instead of introducing fast-travel system.
People thinking that Oblivion's voice acting was beyond worse should play Elder Scrolls Adventures: Redguard. Now that game has some truly horrendous voice acting, perhaps even the worst voice acting ever.