Shades...basically agree (after I said everyone was different...).
The "differing complexity" was reduced in 3E and has been removed almost entirely with 4e (though you could argue that fighters are still simpler then wizards, but that gap is far, far smaller).
This is arguably a drawback.
This reduction is a drawback from three main perspectives:
1) New players. D&D takes time to learn, especially if this is your first RPG ever. AD&D fighters had significantly less rules and base concepts attached to them than any other class, and thus were ideal for beginners. They also gave the beginner an active role in the game even if they weren't very good or confident in role-playing yet: combat was more or less commonplace, and the fighter was active in every combat. Also, the rules and concepts that
did apply to the fighter were the very basics of D&D - to-hit rolls, AC, THAC0, damage rolls, saving throws, HP, role-playing, general in-game concepts and so on; once you learned the basics you could move on to more complex classes such as thieves or rangers, and, eventually, paladins, clerics and mages.
2) Casual players. Some people want to enjoy a D&D game once in a while but don't intend to invest much time or effort into it (such as learning complex rules or spending time studying a wide range of spells). Different complexity levels for different classes meant that the casual player could play and enjoy the game without having to deal with complex rules.
3) Different play styles. Some players like to focus on learning and using the rules (or spells) to their advantage; others like to focus on role-playing (or problem solving, or killing things) without much rule-crunching. Different complexity levels for different classes allowed you to choose how complex the game would be for you.