A number of thoughtful improvements from Morrowind made Oblivion shine, which includes the much appreciated Fast Travel system, where the word "journey" doesn't sound so tiring now. Also, the quest journal is much neater and well-presented, which should be in the first place back in Morrowind's time. In fact, the overall presentation of Oblivion is awesome from gameplay mechanics, user interface, graphics and sound.
Due to the primitive 3D engine back in 2002, though impressive in that time, Morrowind looks flat in its 3D models and textures. In Oblivion, textures are sharper with great lighting to boot, which complements the vast world of Tamriel and its complex architecture. The weapons and gear look detailed and fit snugly in your character turntable. Quality sound effects and dedicated voice acting for NPCs now made the auditory aspect of the Elder Scrolls world complete.
Quests in Oblivion are the main enjoyment here, and are well-plotted generally apart from the main quest, which a lot of fans find themselves lost in exploration and side-questing for hundreds of blissful hours, but thanks to the revamped journal, never totally lost. In fact, side quests are usually the joy in Bethesda games, such as in Fallout 3 and New Vegas. In Oblivion, other than strings of side quests from the guilds enough to form its own campaign, you can self-determine mini campaigns from dedicating your "life" to closing the 60 Oblivion gates to seeking out all the Daedric artifact quests, which some have hilarious quest moments such as disturbing a village with a fake prophecy of calamity from one of the Daedric gods.
The leveling feature is a slight disappointment in the game though, which normally is a progressive and much look-forward-to process in any RPG game, but not in Oblivion. A lot of official guides will advise you to keep your character at low levels in order to have a better chance to clear a certain difficult mission objective, due to parallel enemy level scaling, thus lending a pessimistic tone to the idea of leveling up. But anyway, despite that, Oblivion is a celebrated achievement not just because of its major improvement from the previous series, but because it is a well-designed RPG with an impressive scale and awe to fill for months in a single campaign.