Talking production dirt bikes:
First bike to use a front disc brake? Rokon c1974. Common: KX125/250/500, 1982.
First bike to use a rear disc brake? Rokon c1974. Common: KX125/250/500 & KTM125/250/300/350/500, 1986.
First bike to be water cooled? KTM125, 1980. Common: YZ125/RM125/CR125/CR250, 1981.
First monoshock? Yamaha YZ250/360B, 1974. Common: MX250/400B, 1975.
First linkage rear end: KX125/250/420 1980.
First bike with "safety seat"? YZ125/250/490J, 1982.
first perimeter framed bike? KX125/250, 1990.
first bike to use a cdi?
Common would be a Suzuki, I think.
first alloy framed bike? Honda CR250, 1997.
first fuel Injected bike: ATK604, early~mid 1990s. Its kinda come in in dribbles since, so I'm kinda not sure which mainstream bike to call the first.
First exhaust power valve: YZ125/250J, 1982.
First with >4" rear travel, Maico MC250/400 & YZ250/360B, both 1974.5.
In terms of what's really made a difference in the longer term (although usually not in their debut form!), I reckon the order of importance is:
Long travel suspension.
Water cooling.
Exhaust power valves.
Safety seats.
CDIs.
Front discs.
EFI (on 4-strokes).
Rear discs.
Linkage suspension (although it has packaging/weight distribution benefits, so I'm probably selling it short).
Perimeter frames.
Alloy frames are 50% about marketing, 50% about ease of manufacture, and 0% about performance, so don't even deserve a place on the list....