Worth clearing up the depth of field aspect of Citizen Kane. Greg Toland had done depth of field stuff before, in a naval documentary (or something like it), immediately before Kane. But it had been done even earlier, by Jean Renoir I believe, if not with quite the sharpness and definition they attained on Kane. The way depth of field is used on Kane is poetry, but it was not unique. Depth of field photography was only made possible with improvements in film stock, and with most technological improvements you have a sneaking suspicision that innovation tends to follow technology rather than the other way round. But there's loads of other things which makes Kane really wonderful anyway, so we don't need to worry about it too much.
For starters, the way Citizen Kane tells it's story is elegant and utterly compelling- not just a film about someone, but because it tells it through journalists, it's a film about the intrinsic difficulty of getting to know who someone/anyone is- and of course about the indefinability of any notion of personal identity. Not that it's a riddle, because it's explicit in the film- "maybe Rosebud was something he couldn't get, or something he lost. Anyway, I don't think it would have explained everything. I don't think any word can explain a man's life. No, I guess Rosebud is just a piece in a jigsaw puzzle... a missing piece"- the narrative moves backwards and forwards, and what you know at the end isn't that much different from at the beginning, except now it's poignantly clear that you know how much you don't know about him.
That's what for me is what's so great about the film really. It's a film about myths and myth makers, and the extraordinary gulf between Kane the superconfident newspaper editor and the shell of a man in Kubla Kahn expresses the transience of such myths. This is pretty cool stuff I think, especially in the context of The Hollywood Studio Film which was explicitly designed to martial plot-points towards shaping some sort of conclusion; here the conclusion is ambiguity itself.
Anyway, lots of other cool things about the film. Visually it's extrememly poetic, like that invisible cut from a photo of journalists to the actual group of them- "ideas become reality", ie the very essence of the myth-making CF Kane, and a breathtakingly exciting device (deployed at an appropriately exciting point in the film). Also, the visual world of Kane involves low shots and visible ceilings, something you'd had in German cinema for years; Hollywood films before Kane look flat and two-dimensional compared to it.
It also uses those newspapers and intertitles in an incredibly dramatic way. If you look and listen closely, you can see that the footage is archive stuff, and the reverb on the voice to imitate a loud hailer is a cheap echo, but this makes it all the more impressive an achievement- let's use real documentary footage (I think it does anyway), lets make our own footage look like documentary. Let's create our own self-referential, self-reflexive world.
I think in the end it's a film about story-telling, how many sides there are to every story, and how they don't neccesarily add up to a whole. Yet it does this on an sublime scale (unlike, say, Rashomon). That's a brave and heroic thing to do with a narrative based art form.
Admittedly it's not my favorite film or anything (I'm barely interested in it's political ideas about America) but it's got much that's great about European film, much that's great about American film, a brilliantly ambiguous narrative, found new ways to enrichen film as a text (self-referentiality etc.), there's a great big chunk of Welles' personality, plus it's an astonishing personal achievement for a 25 year old. For that reason it's a massively, massively important film.
For me, the only film which springs to mind as a "great leap forward" like Kane is Rossellini's Voyage In Italy. Both films made what was made before seem prematurely dated.