The buildings might be somewhat more detailed in Oblivion, (not really sure I agree with that) but look at the characters. Alyx alone looks and acts a thousand times more realistic and better than any single NPC you're ever going to meet in oblivion.
Well, now try building an entire set of wardrobes, armour, weapons that she can interchange and try getting her to look natural in all of it, as well as make it interchangeable with every other human character in the game. Now, remove the game scripting and try to add an Artificial Intelligence system that reacts to your behaviour and those around you.
As for the quality of the graphics, my only real complaint is that Oblivion is very 'samey' after a while, but then, if HL2 was freeform, it would probably suffer from the same problem. In Half life, you move from one section to the other, in Oblivion you have a massive world you can wander around in as you wish.
As far as the graphics are concerned, I still disagree, I think Oblivion looks far more atmospheric and generally 'real' than HL2, apart from the HDR, which I always turn off.