Been awhile since I went into design-wonk mode. Buckle up.
1. AD&D is the best designed game in the history of RPGs.
2. It is also one of the least balanced, from a traditional perspective.
3. The player base largely rebelled against the way AD&D *was* balanced, which upset the entire applecart and called for a complete restructuring of the classes in AD&D 2e.
4. By attempting to bolt a new (differently balanced) design philosophy onto AD&D, 2nd edition leeched all the flavor out of the game and did tremendous harm to the brand, requiring more and more outlandish settings (Spelljammer, Planescape, Dark Sun) to try and recapture the flavor AD&D had without theatrics.
To someone steeped in the design philosophy of games like Champions, Magic: The Gathering and d20/OGL/D&D 3e that statement will likely make little sense.
Here's the thing: AD&D has a perfectly harmonious class ecosystem, provided --and here we run into the big problem with issue #3-- you roll 3d6 and require the player to keep the scores in order and enforce all racial limits and class-imposed level limits.
If you make some minor changes to this applecart, for example, allowing players to roll 4d6 and drop the lowest die, you won't see a big effect on the game.
However, this means the dice will determine your available class and even racial options.
Under this system, a Paladin can be pretty awesome, clearly better than the Fighter, and a little better than the Ranger because he's going to be *rare*. The player not only needs to roll a 17, he needs to roll it in the right spot.
Many, many people hated this idea and over the lifespan of AD&D more and more liberal dice-rolling schemes were adopted. By the time we reach Unearthed Arcana, optional dice-rolling systems were presented that absolutely guaranteed you could play the character you wanted (or virtually so).
This starts the entire apple cart wobbling.
It's no coincidence, in my opinion, that when AD&D officially acknowledges that folks aren't letting the rare classes be *rare* (Unearthed Arcana) that things like Weapons Specialization are added to balance the Fighter, new more liberal multi-class options are made available for demi-humans, more levels added to the Druid and so forth.
Beyond changes designed to beef up the basic "default" classes, the classes designed so that EVERYONE could qualify for one of them, Gygax handles this problem through an advanced design technique known as "not noticing".
In short, yes the balance was getting wonkier, the apple cart stacked a little too high but most people seemed to LIKE it that way.
Which is where we started.
In a way it's understandable that the designers of 2nd Edition would find this philosophy unattractive. It basically built power creep into the equation.
But the designers were also afraid (rightly so) of completely tossing out the old, so they tried to build Windows on top of DOS. In the process, they leeched so much of the flavor from the classes that more outlandish settings, and eventually (shudder) kits were added in an attempt to inject a little of the fun back into the mix.
See, there are ways to make perfectly balanced games that are NOT FUN. Bland games are, in fact, some of the easiest to balance.
Eventually the solution was 3e- get rid of the "DOS" understructure that you really didn't want to begin with, make something that's D&D in name only but make sure (to the best of your ability) that all classes and races are balanced so the dice aren't choosing your character options.
This makes 3e a much better game than 2e.
Still not better than 1e- 1e has a kind of magic that defies traditional attempts to balance things. This was the singular genius of Gary Gygax. He could balance things in a way most people didn't like but you had so much playing the game- balanced or not- that no one cared.
EGG realized that games were alchemy.
These days all we have is chemistry. More reliable. Less cool.