Casual Gamer is the lowest common denominator in the gaming industry.
Gamer that put simplicity over complexity.
Most pre-2000 games are propagating difficulty by complexity.
Sadly, the demand for such games is decreasing, as the majority of gamers is getting less and less demanding for intellectually challenging games caused by the intellectual degradation of the society (see "dumbing down society").
Such gamers are called Casual Gamers.
They, as the majority are the pillars of the gaming industry.
As such, the gaming industry is prioritizing their demands and adjusting towards it creating a silent standard or a collective casual gaming model.
Casual Gamer redefined standards as shown below:
Tomb Raider, 1996 -> Complex, Good classical music creating a fantastic atmosphere, gigantic levels, great riddles.
Tomb Raider, 2013 -> Simple level design (a straight line), generic action series music, no unique atmosphere at all, small chambers, very simple riddles.
Even FPS games:
Duke Nukem 3D, 1996 -> Complex level design, big levels, many small riddles.
Duke Nukem Forever, 2011-> level design = straight line, one or two simple riddles, totally messed up weapons handling and energy economy.
Diablo 1 compared to Diablo 3 -> needless to say.