Firstly, what about the ending was really predictable? I doubt you predicted EVERYTHING, and even if you know the ending, it's extremely tense and I panic everytime I watch it. As for what makes the film so great, it's the stunning cinematography, the amazingly crafted atmosphere, great performances, even if Pitt is kind of terrible at times at the end. The script is highly original, and overall, the film is extremely thought provoking, especially if you know Christian theology/philosophy/history, as some of the examples used in the film can be much more haunting that way. The score's fantastic, as well. It's imaginative, and it's become the baseline for almost every cop/serial killer (anything like that) film since, and anything you see as "Oh, everything's done that" it's because Se7en did it first, it's hugely influential, one of the most influential of the 90's. It has a great social commentary on the state of society as a whole, a sort of beginning of the thematic arc of Fincher's films from then to now, with how we're becoming indifferent to the various personal, social, and moral evils in the world, and within ourselves, and how highly selfish we can be.

And finally, Kevin Spacey's relatively brief performance in it is absolutely mesmerizing, and the revelatory climax is terrifying, however predictable it may be, largely because of how it was written, but perhaps more so to his stunning delivery of every line he's given.

I think the film is very close to be perfect, and is a strong 9/10. Amazing.