- no transportation costs
- no unemployment
- barter economy
- 100% fungible labor - farmer, teacher, chemical engineer, doctor - they're all the same and can switch between jobs instantly, and immediately attain the level of productivity within that sector
- prices are 100% determined by labor costs
- demand for various types of items is static, and never changes
- the demand structure is the same across all countries - i.e. if British prefer tea to coffee, we assume all countries prefer tea to coffee
It's not just a "spherical cow" theory, it's more akin to "if a spherical cow rolls frictionlessly down a valley described by a parabola, then it will oscillate forever". True, but useless.
Keynes, as usual, is ahead of the pack - calling out the theory's unrealistic assumptions, and predicting (in 1930s) that not the production, but the interest rates will do the adjustments - leading to persistent trade imbalances and move the economy further from full employment. Which is exactly what happened