Most of what has been said about XP Tables, and classes being unbalanced is just opinions. So take it all as you will.
My advice is that if you are planning to reengineer 1e AD&D, and if you are planning to add homebrew classes to 1e AD&D, you spend a lot of time reading old issues of Dragon magazine dating from the 1980's when people were actually playing and discussing AD&D. You'll get very detailed mathematical explanations of why some classes just don't work, and why some classes overshadow some others. Read letters to the editor. Read articles that attempt to fix the math. Then you'll actually have an informed opinion.
[quoteOne thing to balance Magic-Users that you forgot is the actual acquiring of the spells, which is controlled by the DM. In my 1e/2e games there are no spell fast food chains or unguarded spell books lying around. Also, wizards rarely allow others to copy spells from your spell books for various reasons.[/quote]
In 1e AD&D spellbooks were definitely some of the most important treasure you could acquire. Likewise, scrolls very often ended up copied into spellbooks. But the only way to really control this in the long run is not use NPC M-U's, because as soon as you do, you provide access to large numbers of spells via their spellbooks.
Also with the way spell disruption works, it really can put a hurt on M-Us.
This is certainly true, and is the reason that you can't really make an all M-U party. You have to have a front line of about 2 fighters per M-U.
I disagree on the thief and fighter being overshadowed. If so, why do people keep running them into high levels at my table?
Who said anything about the fighter being overshadowed? Arguably, the fighter and its subclasses are the most powerful in the 1e game. But I'm prepared to prove in detail that the thief has no role in a party beyond color or flavor.
Again, at our table, we use the UA level limits and even then, beyond that you can continue to advance with double XP or Wishes.
Which more or less concedes many of my points in and of itself.
We don't use classes from the UA (except Acrobat), and the spells are controlled by the DM.
Which more or less concedes another block of my points in and of itself.
We do allow Weapon Spec, and have not seen it be overpowered.
In general, a fighter of nth level with weapon specialization will defeat in single combat a thief of twice that level. They'll tend to also have better AC, more hit points, a better chance of hitting, and do more damage per attack. Weapon specialization just furthers skews any possibility of balance in this. But more to the point, weapon specialization tends to nearly double a fighter's expected damage compared to a fighter of the same level without weapon specialization. And in doing so, it ensures that a fighter of a given level instead of taking down a foe in 3-4 rounds, takes down that foe in 1-2 rounds. This is why I frequently say that once you add weapon specialization to the game, it's only a matter of time before the most important roll in combat is the initiative roll. Parties with cavaliers and weapon specialized fighters tend to just wreck monsters of equivalent level. Monsters in 1e AD&D have rather low hit points, and were balanced with certain expectations that did not include weapon specialization. To compensate, DMs generally start throwing monsters at the party that are above their expected level, and probably start cracking down hard on available equipment. But this results in just about everyone, excepting fighters with high CON scores, being glass cannons.
Is it fun? Sure, potentially. I played this way for about 15 years. But is it balanced? Ha!