One thing I got from watching this video of Del Shannon discussing the origin of his big hit was how unusual and ominous that chord progression he uses is. True, he's the one drawing attention to it, but it's commendable because it adds an undercurrent of danger and fear that characterizes the song, underscoring the motif of constantly being unsure of where your lover is.
The other notable things about the song are how he starts off singing like an average teen crooner, but breaks it up by going rough on the "Tears are fallin'" bit, then going falsetto on the "Wah-wah-wah-wah wonder" refrain, showing a fairly full range of tones. I'm always interested in seeing when artists make these kinds of choices. Lastly, that whistling keyboard solo is utterly cinematic, practically Hitchcockian.