Don't think I am over interpreting this song, the man himself said it#Composition_and_writing).
“Imagine no possession” is the end of private property, the most blatantly communist line
“Imagine there's no countries” is the end of boundaries, the end of nationalism, and overall the Alter-globalization (it's worth the click)
“Imagine no religion” is the end of religions, known as Marxist atheism. Communism and actual leftist movements have often been anti religious, and responsible for the separation of the state and the Church in many countries.
“Imagine all the people sharing the world”, self explanatory, John Lennon is calling for peace, and unity. He often delivers literally peaceful messages in his songs (like this one), that could be interpreted as anti-militarism, a subject directly linked to capitalism anyways.
“You may say I'm a dreamer” this line may be deeper than it sounds. He means something important with that: that communist world he's imagining does not exist yet. And that does imply, and he knows that, that countries claiming to be communist are not actually communist. He explicitely says it during an interview (source)#Composition_and_writing): He told NME: “There is no real Communist state in the world; you must realize that. The Socialism I speak about … [is] not the way some daft Russian might do it, or the Chinese might do it.”
What a great man he was… Well aware of this world and a wise man.
Peace and love guys.