i can see the similarities between the two but to really compare them or call it a "poor man's minecraft" is a bit of stretch. yes it's 2d as mentioned, which is a glaring difference which honestly i think is enough to distinguish the two. also terraria is much more combat focused than minecraft.
i put many more hours into minecraft, but i still saw value in terraria. another game along these lines is cortex commander, but that is more destruction and less construction - still worth checking out.
the way i see it minecraft is a creative game, a game for exploring. terraria is a combat game where exploring is secondary, and construction is a tool not a creative expression. at least that's how it was for me.