What makes the music of "1234" so compelling is that its soft, bouncy acoustic guitar backbone perfectly matches the steady bounce of the counting lyrics. Afterward, Feist keeps the verses short and sweet, until the chorus breaks into a lush refrain, complete with a chorus of voices, a plucky banjo, and hand percussion, retaining the song's playful, organic feel.
The warmness of the vocals and instrumentation was achieved by producer/engineer Renaud Letang's use of vintage studio equipment over digital effects and plug-ins: "I record with old desks and old microphones, so everything already sounds fat and warm [for mixing]" (source). In addition, the ambiance of the studio contributed to the warm feel. Feist chose to record at Studios La Frette in the suburbs of Paris because of its natural acoustics. The 100-year-old house's high ceilings and wooden floors created a natural room sound that couldn't be achieved by digital effects programs used in many studios today.
Technically speaking, the song's structure is pretty straightforward, but the arrangements and instrumentation are actually pretty complex and layered. Letang said:
One important aspect of the mix was to take out many things. There were a lot of things playing—for example, a whole brass section—and it had a lot of energy, but it made the track too full. There was too much cream on the cake. It needed to be much lighter. It's a cool song, with a positive and happy feel [...] I wanted to create space around the acoustic music and give the track a modern shape. (Source)
The final result is layered but tasteful, and the plethora of voices and instruments never overwhelm the listener.