As a person who swears in the name of D&D 3.5, I'll have this to say:
AD&D was a more simplistic approach to D&D, with more complicated basic mechanics (THAC0 for example). That is to say, the basic mechanics were more complicated, while the overall approach allowed for lesser variety. The Feat-system I feel is an excellent addition that allows for a more customiasable character and the Skill-system allows for a great variety of situations to be handled in a simplified manner: Want to do craft a basic healing potion? Throw Alchemy skill check, DC 15. Wish to analyze protective magics around an object you're tasked to retrieve? Throw for Spellcraft, DC 35. Want to try and see a weak spot in enemy armor? Throw for Spot, DC 25. Wish to track that wild beast that sauntered through your camp last night? Check Tracking, DC 15. And so on. An easy approach that can be quickly learned and quick to check for: the GM sets the Difficulty Check, you attempt to reach that number of greater by a simple formula of D20+Skill+Skill modifier from abilities and items. Succeed, well done you achieve what you set out to do. Don't succeed, you failed/got the wrong result/followed a false trail/potion does nothing. It also represents the cross-skill nature of all races: not all wizards are poor trackers, not all fighters are unaware of the art of haggling, not all thieves are ignorant in the art of identifying spells from somatic components alone.
The feats add another dimension to this. Playing a mage that was a village alchemist for a in his youth? 'purchase' the 'Skill Focus: Craft Alchemy' feat. A soldier trained to act as a scout? Get the 'Weapon Focus: Short Bow' and 'Skill Focus: Tracking' feats. The feats can represent a minor increase to skills, to a major specialization in the method the character/NPC does their stuff.
The prestige classes can muddle the waters somewhat, there are so many of them thanks to the huge amount of sourcebooks. But they are a more codified manner that represents a significant long-term commitments to a certain branch of methodology within their 'main' classes. Take the Assasin for example. A thief, who has taken an interest in assasinations. Gives up certain benefits from the standard Thief-class, in order to gain more specialized skills and approach, representing the methodological focus of the thief to switch focus from overall shadow-skulking to actual hunting down targets and putting a knife between their ribs or poisoning their food. Their added skills and feats change to represent this at level-ups. And yet, not everyone can become an assasin, there are requirements for taking this prestige class. These represent the minimum ability of the character to make their living (even if only potentially) by this character class and understand and learn the nuances of the prestige class.
Overall, the system allows for a ner unprecedented amount of flexibility and role-playing options, with a system that is at its core simple and quick to learn. The problem is, taht the GM must be aware of the staggering potential and limit it accordingly to their games. Such as they may even degree that no prestige classes are allowed at all, or that some of the more exotic choices for feats and prestige classes are removed.
