Wong Kar Wai has a style of framing a scene that I have honestly never seen from anyone else. He constantly uses negative space, shooting around walls and through small windows. There's an awareness of space in Wong's movies, emphasizing how people move through them, and how our man-made environments bring us together, or pull us apart.
2046 takes those elements, as well as most of the principle actors, from In the Mood for Love and Chung King Express. But where those movies were more focused, and their characters more distinctive, 2046 drifts from place to place. It does so beautifully--the cinematography lingers on the rich surroundings, and its color palette is saturated without being garish--but it doesn't really satisfy.
Theoretically, this is a follow-up to In the Mood for Love, taking place after Chow (Tony Leung) has moved on from his affair with a next-door neighbor in the previous film. He's now a playboy, sleeping around with a number of women. The scenes are shuffled, and interspersed with a sci-fi story Chow has been writing, about a man on a train to "2046" who falls in love with a broken android. Somehow, this story is meant to be tied to Chow's various flings, as he becomes involved with his new neighbor (Zhang Ziyi), remembers a past relationship with a professional gambler (Li Gong), and flirts (though without much heat) with his landlord's daughter (Faye Wong).
All of these are fine actors--Faye Wong in particular stole the show from Chung King Express, and Leung is usually convincing for both his comic and dramatic roles. But Wong Kar Wai seems to have locked almost everyone down into themselves for 2046, as if he's less interested in them as characters than as static elements for his composition. Zhang Ziyi is allowed the occasional character quirk, and Chow's editor Ping (a phenomenally ugly man) lends energy to the few minutes he has on screen. But the rest of the time, these characters are too bottled up for anyone to care--there are no cracks in Leung's debonair mask, and as such it's hard not to feel a little repulsed by him, if you feel anything at all. In the end, we're left without much character, much plot, or much spark.
Long story short: 2046 is a pretty slow two hours to look at beautifully-shot hotel rooms.