Oblivion was and still is a beautiful game, in my opinion. The landscapes are vibrant and beg you to explore them, there's a lot of freedom, lots of character customization, a good soundtrack(though probably not quite as good as Morrowind's). With all of the good things said, I definitely had my gripes with the game. For one, I dont, and never did like randomly generated dungeons. I realize the developers include it so the player experiences something new each time they enter a dungeon, but for me it takes away from the experience. Why not include inaccessible areas that you can only get to later, instead? That gives you an incentive to go back to the dungeon, and with new areas to explore instead of having something different solely because it's generated with a different layout.
The scaling of monster levels is also a pretty bad idea. I always enjoy the feeling of discovering a new area, meeting the menacing creatures it has to offer, and being knocked around to the point of realizing my own mortality the hard way. I level up, come back and taunt them with my own combat experience.
I know that finding better armor, stronger weapons, and learning more powerful spells counts collectively towards your character's finesse in combat, but the feeling of accomplishment is less substantial after leveling up for hours on end.
The fast travel feature, although entirely optional, sucks a moderate bit of the fun out of exploration. Sure you can't fast travel everywhere, but even to those places where you can, you would always have more fun discovering the area on your own and being able to fast travel from that point forward. This is probably my smallest gripe with the game, though, seeing as how you can only transport to the more major locations.
And the Oblivion realm. Yes, the entire centerpoint of The Elderscrolls IV itself, is probably one of the most irritating concepts in the game. The hellish area is entirely too repetitive, and going through a gate to lock it often really gets old after a few times. This is definitely the biggest aspect that I dreaded about the game, after around my 4th time and 2-3 minutes after walking into the gate I tell myself "Ughh ive been here done this already. At least make each subsequent visit different enough to capture my interest." But nothing ever changed, alas at least I felt great after locking each gate.
With all of this said, im not sure if I could call the game dated, at least it isn't dated enough to drag it down a lot. It runs off of the same engine and is slightly less prettier than Fallout 3. Though, this is entirely from a technical standpoint seeing as how Oblivion, in my opinion, has a much better art direction. Like you pointed out, Oblivion is colorful and not a dull wasteland as in Fallout 3. But the biggest errors in the game have little to do with it's age and more to do with sour ideas on the Bethesda's part.
