A resistor in series with a motor reduces the current. But when a motor starts running its current is very high so the reduced current will not allow it to start running. A diode does not reduce the current much, it reduces the voltage by a fixed amount instead. Then with the voltage reduced its speed will not be too fast.
The threshold voltage of a Mosfet is when it has a very low current and is almost turned off. Its "On Resistance" shows the Vgs it needs to be turned on.
The graphs on a datasheet are for a "typical" one that you cannot buy. Some are minimum and some are maximum. If only Mosfets with a threshold voltage at the maximum of 2.4V are available then their current will be fairly low with a Vgs of 3V.