Oldboy is in so many people's Top 10 and I've heard so much praise and been pestered to see it forever. So, needless to say, the expectations were perhaps very much too high going in. I did rather enjoy the first half. The plot was certainly original to start with, and the cinematography definitely ranks quite high on the visually gorgeous side of things.
My problem is that I usually have an annoying talent of spotting where the plot is going quite early on (yes, I'm one of those 'I bet he killed her' people, which annoys others who might have wanted to stay in suspense until the final showdown. :P), and with this plot it was just glaringly obvious to me where it was going. So for most of the film I was just waiting for the plot to catch up and put me out of my misery.
I also think that after showing so much promise at the start, and having gone to all the trouble of trying to confuse the watcher, the actual underlining story was way too simplistic and insipid. I was desperately hoping that I would get surprised at the end, which didn't happen. Having said that, I can certainly see why people rank Oldboy so high. It's definitely head and shoulders above the standard of a stereotypical Hollywood production. And it does look very good indeed. I think it suffers from building up too big expectations without delivering the depth of a masterpiece that it aspires to be.