What are the hallmarks of 90s cycling?
Here's my 90's bike - the main period features are


Earlier alloy construction, with fatness in the downtube.
Group set, Shimano RSX and exage brakes.  The RSX brifters lack trim support, so badly set up transmission makes a takka-takka-takka sound when in bad combinations.  So 90s.
Too new for sissy brake levers, too old for cyclocross interrupter brake levers
Seat post diameter of 26.2 mm (because 25.4 mm was too small)
No frame pump mounts - they're 80s.




Unsure - 


This bike still has a quill stem and a threaded fork, which would have been old-school even at the time.
Pedals - the period originals weren't with it, but Look clipless would be right.  Platforms are probably okay too, but SPD would be seen as too much MTB.
Seat - this one was probably a replacement.
Rear wheel cutout - this bike looks a little tracky/tri because of how the rear wheel tucks into a cutout in the seat tube.  This is not a 90s thing, its an overall design thing.
FD connector - this one has a braize-on mech rather than a band-on one, due to the cutout.  Is this a 90s styling?  I don't know.


Please make comments to refine this answer.

EDIT I think  @ebrohman may have identified some of the defining style characteristics of a 90's bike.  


Ugly - case in point
Minimalist - nothing that's not needed for going fast
Materials - There's nothing wrong with a good steel frame, but by the 90s steel was the cheaper end of the market.  Aluminium alloy, and titanium had their blips, and right at the end of the 90s is when carbon was coming in.  
Unconventional - The standard diamond frame was pretty much defined, so new materials and the new possibilities then became available.  Consider my bike's brutal downtube, which lead on towards hydroforming of aluminium we see in today's MTBs.