I'm not sure I would characterize 1st Edition AD&D as "less complex". In many ways the evolution of the D&D ruleset has consisted of removing unnecessary complexity to make the game easier to manage. For example: a standardized simple formula for computing attack rolls instead of pages and pages of "to hit" tables (I remember when bootleg copies of the tables in the 1st Edition DMG were passed around in my school because the book had just come out and not everyone had it yet); standardized experience points to gain each level instead of class-specific; a more standardized pattern to class progressions instead of a hodgepodge of ad hoc rules; standardized mechanics like skills and feats instead of class-specific ones.
I'm not sure I would characterize 1st Edition AD&D as "less complex". In many ways the evolution of the D&D ruleset has consisted of removing unnecessary complexity to make the game easier to manage. For example: a standardized simple formula for computing attack rolls instead of pages and pages of "to hit" tables (I remember when bootleg copies of the tables in the 1st Edition DMG were passed around in my school because the book had just come out and not everyone had it yet); standardized experience points to gain each level instead of class-specific; a more standardized pattern to class progressions instead of a hodgepodge of ad hoc rules; standardized mechanics like skills and feats instead of class-specific ones.