Classes :
Assassin - unchanged
Barbarian - as the vanilla game but with the following changes : Charisma score adds to Rage strength and Con increase, 10% Damage reduction to physical attack, additional 10% DR and movespeed at level eight
Bard - unchanged
Cleric - must have a correctly spelled deity in the character sheet and domains must consist of those allowable for priests of that deity
Druid - movespeed increases while in non-town areas, wildshape and Animal companion altered to underdark theme, must have a nature deity entered in character sheet
Dragon Disciple - unchanged
Pale master - improved undead summons and Str and Con boost, negative energy resistance, pos energy vulnerability
Ranger - as Druid and increased Search Spot and Listen at level five while in wilds, FAvored Enemy gains a to hit bonus in addition to the vanilla games damage bonus.
Rogue - some changes to stealth and Use magic device. Stacking traps and outright killing a PC with multiple high damage traps is not allowed.
Sorceror and Wizard - replaced normal familiars with Underdark ones
Fighter - unchanged
Weapon Master - saves Bonus, Bonus cleave feat, Bonus Blinding Speed feat
Shapeshifter - may become any race in the module
General Notes -
All PrCs must be applied for and worked toward in game
All summons spells are changed, have a much shorter duration than the vanilla games 24 hours, and will be altered depending on your cleric domains and summon reagent used, summoning undead may be considered a crime
Many spells are altered in most cases tweaked to become weaker or more flavorful.
Spell Focus : Abjuration now affects the strength of spells cast from that school.
Bards, Rangers, and to a lesser extent Barbarians can expect to encounter a lot of class-specific items
Wands purchased from PCs are very helpful in surviving and you forfeit being able to use them if you play a pure barbarian or fighter
Craft Weapon and Armor now do nothing. Craft Traps is used by Dms when applicable to determine how knowledgeable and handy your character is regarding machinery.
I think I've about covered it, others may correct or add more, hope this helps.