Citizen Kane

We watched Citizen Kane yesterday (with a bottle of Minnervois, Collines du Paradis, a red wine which I can only recommend).

I liked the film. But I keep thinking that I have missed something. I am no cinema connoisseur and although I liked it, I would not rate it as one of the best film I have ever seen. A recent survey rated it in the top 10 of all-time movies.

Is it because of technical innovations, use of photography, light, etc...?

Please tell me what I should have looked for and did not see!