Perhaps the biggest change is a complete overhaul of the skill system, which borrows heavily from a system suggested a few years back at A Rod of Lordly Might. More than half the usual 3.5 skills are gone or collapsed together, leaving behind Alchemy, Acrobatics, Combat, Craft, Handle Animal, Survival, Stealth, Thieving, Ride, Swim, and Use Magic Device.
Most important to note is that there are only four ranks for each skill: unskilled, skilled, expert, and master. Most races and every class in the campaign starts out "skilled" in at least one skill:
Beastfolk - none
Dwarf - Craft
Elf - any one skill
Gnoll - none
Gnome - Alchemy or Use Magic Device
Goblin - Stealth & Thieving or Survival
Hobbit - Acrobatics or Stealth or Thieving
Lizardfolk - Swim
Scorpionfolk - none
Bard - Use Magic Device
Cleric - Alchemy
Druid - Handle Animal
Fighter - Combat & Craft
Monk - Acrobatics
Paladin - Combat
Ranger - Acrobatics, Handle Animal, Stealth, & Survival
Wizard - Use Magic Device
An elven cleric of the Blue Moon makes use of her skill in Alchemy.
Ratbagg the goblin barbarian, for example, is skilled in Acrobatics and Survival (barbarian) and also Stealth and Thieving (goblin). As detailed on the post over at Rod of Lordly Might, training in skills is unconnected to XP or character level - instead requiring you to train the skills outside of adventuring - and there are no DCs to meet. Rather, you succeed at using a skill (if a roll is need - see below) on a 5+. The dice you use depends on your skill level:
Unskilled, d6; skilled, d8; expert, d10; master, d12.
Modifiers will be applied on a contextual basis: if you want to use Acrobatics to do a backflip, you might get a bonus for high DEX. Using Acrobatics to do a standing jump upwards, however, would be more likely to get a benefit from high STR. Size, environment, and conditions are also considered; and if someone might interfere with your action, you make an opposed roll.
For example, Ratbagg attempts to sneak past the ogre guard. Ratbagg is skilled at Stealth, is also small, and is wearing no armour - he can roll d8+1 for his check. The ogre is unskilled, and has also recently overeaten and is drowsy - he rolls d6-1. Ratbagg rolls a 4 for a score of 5, and the ogre gets a 1 for a score of 0. In this instance, Ratbagg not only succeeds, but because he succeeds by 5 or more he's achieved a major success. If he wants, he can try to pickpocket the ogre as he goes past, with a bonus - the ogre's obviously almost asleep, so it should be a piece of cake.
Well worth noting is a change which runs counter to the "round peg in round hole, square peg in square hole" mentality behind Type IV's (and to a lesser extent Type III's) take on party balance: anyone can use any skill, even without any training. Being unskilled in Ride, for example, means you can ride a horse, albeit not very well - higher ranks of Ride will increase what you can do with your horse, but the bottom line is that no character is excluded from any option available to him or her on account of skill ranks.
I like this system because it retains the liberating freedom from skills that you get in OD&D, while still providing a (ridiculously simple) mechanic for resolving opposed skills and determining the relative success of an individual action. Indeed, in some cases, it's even more freeing than old-school gaming: even if you're unskilled in Acrobatics, you can climb an easy wall without having to roll, and will succeed at a trickier climb 1 time in 3. This gives adventurers more options and agency - and suspends belief a wee bit further - than the situation where only thieves learn how to climb in adventuring school (and even then will fall off a training wall 15% of the time).
Over the next few posts I'll talk about some other house rules: my replacement Fighter class, which was made necessary when I decided to remove feats from 3.X; and rules for armour check penalties and encumbrance. And maybe some other, equally exciting, stuff. But I'd also like to talk, on the back of this fascinating and valuable archival post over at Papers & Pencils, about the rocky relationship between D&D and conservative Christianity.
D&D terminology explained:
XP - experience points. Characters get these by killing monsters and finding treasure. When they've got enough XP, they "level up" and gain more powerful abilities.
DC - difficulty class/check. This is a number which needs to be rolled equal to or over to succeed at whatever the character is trying to do.
STR & DEX (also CON, INT, WIS, & CHA) - Strength, Dexterity, Constitution, Intelligence, Wisdom, & Charisma. These are the six attributes used in D&D, and they determine your character's capabilities (as well as career options). Fighters often have high STR, for example; Wizards usually need high INT.