From a post on 7-10-2004, this weekend addressed many of these...
Here's a quick recap of the pitfalls identified thus far in the thread...
Pitfall: The number of attacks rolled at higher level becomes difficult to manage (with multiple heroes and villains).
Pitfall: Instead of spending time doing more complex character development, playtesting an adventure, or writing additional background for the campaign or characters, the high level DM is forced to "build the adventure".
Pitfall: A noticable absence from most high level modules (12th level plus) the aforementioned "stat blocks" with different levels of buffs. By "building" they must stat out on an extensive spreadsheet, each encounter so that it is ready at game time.
Pitfall: There are too many types of modifiers, with differing degrees available for different modifiers, which becomes very difficult to track. Stacking/Non-Stacking works to place necessary ceilings, but may cause more confusion than it’s worth. (It may also grant too many bonuses at lower levels.)
Pitfall: Although there are some class combinations that work at higher levels of D&D, not all classes are built equally.
Pitfall: Not all prestige classes build effectively (caster level is king).
Pitfall: There are too many enhancements, and stacking becomes overly cumbersome: 17 enhancements to BAB, 23 enhancements to armor class, 16 enhancements to Base Saving Throws.
Pitfall: There are 45 unique "tactical" conditions, each with unique modifiers to combat. At higher levels, you are exponentially more likely to require multiple tactical conditions, affecting a variety of the players and villains during a combat.
Pitfall: There are a variety of feat and class abilities that can affect combat, each with unique modifiers (or variable modifiers) to combat. At higher levels, you are exponentially more likely to encounter multiple feat/class modifiers, affecting a variety of the players and villains during a combat.
Pitfall: What do you fight above 12th level that doesn't have AT LEAST a 10' reach?
Pitfall: The combinations of types of defense and modifiers to defense are challenging to track, and become exponentially more difficult as you go up in levels. (Examples: AC, Touch, Incorporeal Touch, Concealment, etc.)
Pitfall: Remembering and adjudicating the ways different defenses interact.
Pitfall: Combat at higher levels becomes about identifying and exploiting your opponents weakness, and not about exercising your character’s abilities. By epic, defenses become so strong, it is very difficult to affect opponents.
Pitfall: Armor’s Dex Bonus Maximum isn’t necessary.
Pitfall: The dramatic increase in effectiveness that the right enhancement combinations can have on high level characters can be overwhelming to manage as a DM, and anti-climactic in game play for the characters (Divine Favor, Righteous Might, Assassins Senses, Keen Weapon…). Further, characters become overly dependent upon their enhancements, and one effective dispelling of enhancements can be overwhelming to the players.
Pitfall: The average player is horrible at keeping maps.
Pitfall: As players are granted access to extremely powerful movement spells, items, and abilities, mapping becomes an enormous waste of time/resources. Dungeons/Encounters above 12th level need to each present considerable magical AND natural obstacles in order to not provide a well-played group with a significant tactical advantage.
Pitfall: AOO's. The whole system, especially at high level. 5D movement (3D+invisible+ethereal), most creatures have reach 10' or 15' plus, and keeping track of multiple AOO's.
Pitfall: Wizards in armor. Another mechanic. Another roll, more complexity.
