A fun - yet fundamentally flawed - roleplaying game, Oblivion is far greater than the sum of its broken pieces.
The core of Oblivion can be summed up as follows - you create a character of your own design and explore a relatively large world at will, completing quests and killing monsters and building your skills and acquiring new items. Sounds like pretty much every other RPG ever made? Yes, but Oblivion earns the vague description, as what you get out of the game is also related to how much you put in. Added to the flaws mentioned before, that's an awful lot of effort for the player to exert.
On the presentation side, the graphics are quite nice outside of the occasional janky texture and some truly ugly character faces (these can be alleviated somewhat with mods) and the sound effects and music are all passable. It's certainly not a well optimized game by any measure, either, but for the most part you aren't going to live or die by load times or the occasional framerate hiccup so it ultimately doesn't matter a whole lot. Outside scenery is often beautiful to look at (again, this is greatly enhanced with visual mods), particularly during dusk and dawn.
So where in the experience of playing Oblivion might you be driven nuts?
The world is not particularly interesting. Maybe it's unfair to make direct comparisons to Morrowind, but for an older game with downright crude graphics by today's standards it manages to throw up a great deal more variety in both locations and characters. Sure, Bruma has snow and Leyawind has sort of a swampy look going on outside, but that's about it. No desert or badlands, no real plains, though the world of Oblivion makes for a passable substitute for the volcano enclosed by the Ghost Gate. Unfortunately, all of the cities are pretty dull. Architecture is different, but subtly so, and while the capital is easier to navigate than Vivec ever was, it's far less visually interesting. Everything reeks of your standard European medieval setting with no chances taken. Even the dungeons are all variations of the same four setpiece themes - Ayleid ruins, cave, mine (basically a modified cave), and fort.
Everything is scaled. In interviews the reason for this is to keep the game challenging, so that the player doesn't go and outgrow everything. So what? You won't get any decent equipment until you start hitting later levels, regardless of what remote location you explore. All of the loot is randomized and scaled by level. About the only challenge you face is attempting to come out on top of the still needlessly painful leveling system. Ten points in any major skills levels you up, but you also receive bonuses if you've leveled a certain number of skills in a particular attribute. Say you want to raise endurance, strength, and agility, and you've got five points in blade and five points in heavy armor that will level you up the next time you sleep. That's 2 to strength, 2 to endurance. You could train an extra couple of points out of a minor skill from a trainer, stand in one place and hop up and down for half an hour to build up your acrobatics for agility, and maybe whack rats with a mace if you have blunt as a minor skill. You'll waste a remarkable amount of time trying to get your three +5 stats each level. You don't have to, really, but you're only crippling your character if you don't. This was broken in Morrowind and it's more broken now, given the limitations they've placed on trainers.
The quests suck. The main plotline is terrible, and both Patrick Stewart and Sean Bean manage to sound bored out of their skulls. The sole redeeming questline is that of the Dark Brotherhood, where you play as an assassin (some of the later Thieves' Guild quests aren't bad either). All the rest are fetch quests or 'go here and kill X monsters,' especially those found in the Fighters Guild. Amusing still is the fact that you can become head of every single guild even if you have absolutely no talent in that area. Want to slash your way to the top of the Mage's Guild? Go ahead - it's absolutely nonsensical but you can do it, and it's easier than using magic as the game strongly favours melee characters.
It's impossible to create a character that doesn't look like a troll. So many facial settings and they all result in varying degrees of ugly.
Other than the named actors, three or four dudes voiced every single character in the game. Every redguard in the game (good, bad, enemy, main plot character, whatever) has the same voice. Two women voice every female character (though it may just be a single actress who speaks through cotton balls to make the dark elves' voice).
They took out the Levitation spell. Not cool.
The menus are also garbage, even when tweaked via mods to fit more than four items per page. They're the most obvious parts of a joint PC/console development, although even when patched the game does not work properly with an Xbox 360 controller, which would be an excellent fit with the game. Go figure.
And yet in the face of all that the game still manages to be fun. It's kind of neat wandering the countryside looking at the scenery, even if it isn't all that varied. And while there's no lure to discover remote dungeons for the sake of finding unique loot, it's nice to stumble across some random fort in the corner of the map. Charging up a strong attack and sending smaller enemies flying through the air is great fun. You can still do so much stuff in the world that it's amusing enough just to wander around, even if each bit of the experience is flawed. For all of the things Bethesda fixed from Morrowind, it's also served to dumb the game down somewhat in terms of its overall scope.