The difference in feel between chain drive and DD drive is due to the cam (or sprocket), not the chain itself. The cam is also the source of power for chain drives - it acts as a multiplier on the pedal's "throw" - which is why DD doesn't produce the same level of power.For all practical purposes with average spring tension and average beater weight, skilas is correct that the chain will never appear slack. But to say that the chain is always under tension means that it should have an identical feel to a direct drive, which it doesn't so that should be enough evidence to convince someone. If tension is the only force on the drive system, they would operate and feel identical.
The cam is also why chain drive is easier to control.
And finally, the cam is also why DD feel more readily responsive while chain drive feels slower to respond to input. There's more ground to cover as the chain travels up and down the cam. The difference in feel has NOTHING to do with "slack" in the chain. It's all about cam vs no cam.