It's hard not to make the immediate comparison between The Curious Case of Benjamin Button and Forrest Gump, given that the two films share a writer and, at first glance, apparent thematic similarities; in a critical world that generally gives pride of place to the director-as-author, and which lauded David Fincher's previous film, Zodiac, it's interesting that the director comes across, in many a critical commentary, as a minor player on Benjamin Button. As my wife noted, though, the Forrest Gump comparison could easily be made with more or less any film that covers a significant span of the twentieth century (even a film made well before Robert Zemeckis's film, and in a very different context).
Where Forrest articulates little sense of why he does what he does, Benjamin's desire to go out into the world is a conscious choice, and he is aware of the ramifications of his own choices - he's no naif, and he acknowledges his own failings, at least in the torch-carrying department. He, too, is a participant in history, but the only president we see here is Theodore Roosevelt, long before Benjamin is born, in an opening anecdote about time and the inevitability, sometimes ugly, of change (that anecdote, like the later sequence explaining an accident, or the running jokes about a series of lightning strikes, recall Paul Thomas Anderson's playful narrative strategies, particularly in Magnolia).
The film is suffused with a remarkable sense of nostalgia, not just nostalgia for the lives the main characters might have led in different circumstances, but nostalgia for worlds gone by, for memories of youth, even for the objects that accompany a life: shots of framed pictures in a New Orleans home, for instance, with all the weight of personal history such images may bear. The inevitable tick-tock progression of life is set in yet sharper relief by Benjamin's unusual aging process, and those moments where he seems to overcome his unusual station in life, in the central romance but also, beguilingly, in an extended sequence in Murmansk with Tilda Swinton (how many films can there be that cast that city in such a glow?), are intensely sad even as they represent Benjamin's sweetest memories. Even the framing device, set against the backdrop of Hurricane Katrina and inevitably somewhat obvious, is still of a piece with the film's great theme of inevitable decline, of the impossibility of fighting against elemental forces; the modern sequences are filmed in radically different shades, abandoning the warm browns and yellows of the past for the harsh surfaces of a modern hospital (the old nursing home is long gone).