Musically, "Imagine" is very simple (but then the song's point isn't virtuosic playing!),
(the high-low high-low high-low sound) plus
broken chords (the one-two-three pattern). Those are both very basic piano-playing techniques, but I think the progression of
split, split, split, split, split, split, split, broken,
split, split, split, split, split, split, split, broken...
is still fairly distinctive to that song, and even if you try playing it with different actual chords, people will think "Ooh, sounds like 'Imagine'."