Despite his waspish wit and the armour-plating within which he emotionally coccooned himself, John could open up in song and bravely reveal his vulnerability more candidly than almost any other artist. "Julia" is a good example of this. Like so many of his songs, it's very heartfelt, direct, personal and rather beautiful in its simplicity.
Taken in isolation though, literally just as a song (lyrics delivered vocally set to a melody with instrumentation) it doesn't do much for me at all. Appropriate for the track I suppose but those wispy, gossamer vocals are almost too irritatingly wishy-washy lightweight and the tune itself is wafer-thin. Like "Dear Prudence" it lacks the meaty vocal bite we had become accustomed to from Lennon.
As a classic slice of soul-bearing and in the guard-dropping context of its author's reflections on his fractured relationship with his mother (and subsequently the presence of his older muse) it does make sense. But it would be bland if not to say meaningless, in my opinion, if covered by any other artist. A song which for me carries no validity unless John Lennon happens to be singing it.
I'm afraid it would be among the first to get the chop were I to play the "what if the White Album was a single album" game with which we are all familiar.