In this section various rules and aspects of the game will be discussed.  Many disagree with what I have to say here, but each to their own.  DM's are free to change and create what they wish within their worlds and campaigns, so I used that freedom to make some interesting changes and additions...

1. The first thing I want to discuss is the wizard class not being allowed to wear armor.  Many feel the reason is because metal interferes with the flow of magic.  Not so, and it even states that in the books.  The reason for this basic rule is that wizards are required to get into various poses and postures for the somatic component of spell casting and armor would hinder the necessary posturing.  No problem...except when dealing with leather armor.  If thieves can climb sheer surfaces, cliffs, walls, etc without the aid of rope or climbing tools and suffer no penalty while wearing leather armor, wizards should have plenty of range of motion to cast spells while wearing leather armor.  Another point in favor is that elves wearing elven chainmail as well as elven plate can still cast spells, and leather is alot more flexible than even elven chainmail much less platemail.

 2. The second thing is the maximum number of spells per level that you can copy into your spell book based on your intelligence as a wizard.  For starters, this rule is completely optional and therefore, I do not use it at all.  It's ludicrous!  Wizards are already limited by spell progression.  Spell books should be able to hold as many spells as possible, regardless of intelligence, so long as there enough pages within the book.  Example:  here you are, brain genius wizard with a score of 18 and you can have 18 spells per level according to the rule.  You already have 18 in your book.  You come across another first level spell...you can read it, you can cast it, you even know how to write it, but wait, no, you cannot have any more spells because why?  You have 18 already?  Who gives a shit!  Copy the sucker and memorize what you can according to the spell progression table.

3. This is the clincher! Suppose you have a thief and a fighter, both with a dex of 18. That gives a +4 to AC bonus,right?! The thief wears leather armor while the fighter wears full plate. That is a 7 point difference in armor class. Their total AC with these figures only would be 4 for the thief and -3 for the fighter. If your THAC0 is 20, what do you have to roll to hit the thief? 16 or better... What do you have to roll to hit the fighter? A perfect 20! But wait...that's only 4 points difference on the dice even though there are 7 points difference between the armor classes. That's not fair to the fighter! What I do to compensate for this numerical phenomenon is to deduct 1 point of damage taken by those who can only be hit on a 20 for every point of AC over the roll of 20 that is need. In other words, the above fighter has a total AC of "23" (THAC0 needed = 20 + AC points leftover which = 3). He would take 3 fewer points of damage (none below 1) from someone who had to roll a perfect 20 to hit him! This also negates the critical strike on a 20 as well because it is the only thing they can roll to hit him.
If you don't follow, THAC0 is "to hit armor class zero." His AC is -3. He has 3 points below 0 and since the opponent's THAC0 is 20, he takes 3 fewer points of damage from that opponent.