Being able to make whatever chapter you wanted and tailor it to you defined what Chaos was all about until Games Workshop mutilated the Codex.
In the HQ section, very similar to what you see today, you can take 0-1 for an HQ. In the elites section our Chosen back in the day you could have a special weapon for every 3 of them. Nowadays, it’s not so great to have so many special weapons because one D weapon can take them all out. The Possessed weren’t that good either back then, BUT arguably they were better that they are today. The Obliterators, being toughness 5 where also really good as well.
In troops section, you had your troops and then a special elite for each good, but if you bring an HQ marked with a Gods Icon then all your special elites would become troops. NIFTY!
Chaos Raptors were the first unit with hit and run I believe. Chaos Dreads where terrible even then, either way I feel like it the armored walker model mechanics needs to be universally reviewed by GW. Great models, they inspire people to literally start playing the game, but sooooo frustrating on the tabletop.
The first version of the Defiler was in here too – it was the coolest thing ever with it came out. However at the time all we had was this picture, and a dream to convert it! Remember, back in the day Games Workshop would release the pictures before they had the models done and people where just going ham converting up their Defilers using all sorts of bits from Necromunda to buzz saw arms and GorkaMorka and NO ONE had any idea how big this kit was going to be.
When they finally released it at Games Day people lost their minds.
Then we get into the specialized units, like Alpha Legion which let you apply special rules to it. The Alpha Legion couldn’t work with daemons as much but they could always infiltrate for just a couple points. Iron Warriors where a thing, people were afraid of them because you could bring an extra heavy support and it unlocked Obliterators into the elite slot. Then they’d get a vindicator and a basilisk, (yes the imperial guard one- in 2003 that was a big deal). Oh and they could take a servo arm too, you know just for fluff…
With the Mark of Khorne, you could get +1 attack and had to attack the nearest unit so you could kinda lead them around by the nose a bit, it was a bit funny. Sweeping advance was also a thing so if you won an assault you could then get locked into another combat and just stay in assault. It was a really interesting time for Khorne players; it’s probably their finest hour.
In the book of Nurgle, it was all pretty standard, one thing of note was they had a rule called True Grit which meant they could take a Bolter and a close combat weapon and the bolter counted as a one handed weapon. They had their own psychic powers which we haven’t seen since then.
Then there is the cute little tactics articles, as they outline how the studio thought the army should be played. Well I can tell you that at no time, at any tournament, did anyone ever play it this way. It was almost laughable how out of touch these “tactics” were with what you could create with the book itself.
Checkout the full color hobby articles and pages towards the back. This section, as you can tell, was really what Games Workshop really wanted us freaking out over.