Final answer:
"Eleanor Rigby" stands out from other late '60s rock songs mainly due to its unique orchestral arrangement, which relies on a string quartet rather than electric instruments common in the rock genre.
Step-by-step explanation:
The instrumental aspect that makes "Eleanor Rigby" unique in comparison to other late '60s rock songs is its orchestral arrangement. Unlike many rock songs of that era which heavily featured electric guitars and drum solos, "Eleanor Rigby" by The Beatles is noted for its string ensemble, which lends the song a classical and somewhat somber tone. This use of a string quartet in a pop music context was innovative for its time, setting the song apart from the common use of electric instruments and rock-driven elements.