Adventurer Conqueror King System (as voted for by my friends at Google+). I figure I'll attack the book pretty much in order, passing over the introduction and moving on to section 2 - Characters. (My quick mini review of ACKS is here).
The default method of character generation is definitely "Old School". 3d6 IN order! An included optional rule allows one to roll up additional back-up characters to insert in the campaign when your current character dies (these characters can also be used by the Judge/GM as NPC in the meantime). Yeah, run as written, even the character generation rules hint that it's a fairly lethal system ;)
Stats are listed in the "Old School" order of Strength, Intelligence, Wisdom, Dexterity, Constitution and finally Charisma. This is still the way I list my stats unless I make a conscious effort to do otherwise. It's nice to have a system that matches my ingrained memory.
Hit Dice for the character classes range for a D4 for the Mage to a D8 for the Fighter. Classes hit their max at 14th level and reach their last class title at 9th (ex: Warlord for fighters and Wizard for mages) and that's when HD max out. THAC0 is slightly slower the AD&D (probably in line with B/X, but I can't confirm as they are packed away). Saves are the typical 5 categories you see in the OSR. All classes get proficiencies, which can be class based or general. I'll focus on them with a later post.
Fighters are pretty basic. They (as well as the other classes) will differentiate or specialize by their skill selection. Mages can cast spells up to 6th level (upon attaining 11th). Clerics can also cast spell of up to 6th level, and do not get 1st level spells until reaching ins level cleric. Thieves get he usual list of thief abilities, successful use of which requires the roll of a d20. The abilities are purely level based, there is no point pool like in some variations in the OSR.
Assassins fight as fighters and get HD, backstab and some stealth abilities as thieves.
Bards are masters of lore and can inspire courage in their party (bonuses to most rolls). They can also dabble with magic items usable only by mages, read languages and inspire loyalty in his henchman and hirelings at 5th level.
Bladedancers are a female cleric / fighter class. d6 HD, turn undead, cast spells (limited to 5th level spells) and fight like a cleric. Unlike clerics the can only use slashing and piecing weapons and are lightly armored.
Explorers are "almost rangers". They fight like fighters, have a D6 for HD and get stealth abilities outdoors. They are also useful in keeping the party from getting lost.
Alright, next up (maybe even later today if I can squeeze in the time) I'll discuss the non-human classes.