Parts are compatible, and the underlying system hasn't change, but Technic has transitioned away from using traditional studs, so you may find it a bit clunky to integrate older components.
The biggest difference for motors is electrical compatibility, as noted in another answer. Both the wire connectors and voltages used have changed a few times over the years.
The Technic axle connection has not changed, so you will be able to connect any LEGO Technic motor to modern gears without difficulty. It may be difficult to integrate older motors into studless Technic builds, as most were designed to attach using studs rather than Technic pins, but this is more of an annoyance than a hard limitation.
Another significant annoyance may be the gearing in older motors. Most older motors simply provided a Technic axle connection for a standard DC motor. Direct drive from a DC motor is too fast for most applications, so many older sets included multiple levels of gear reduction before driving axles on vehicles. Here's an example from 8054:
Newer motors are typically geared down, and may be used to drive axles directly, as seen here on the Batmobile:
You can likely work around the gearing issues, but be aware that you may need additional space and gears when using older motors.