So you've been thinking of starting a new army and are giving Chaos a look. What are the possible draws to this army?
The background of the different groups of Chaos Space Marines presents several layers, any of which can be appealing on their own or in combination.
For the rebellious-natured, the whole notion of fighting against the monolithic Imperium is appealing. Why defend such a rotten status quo? The 'dark reflection' of the flagship army can be a draw as well - corrupt instead of stalwart, unpredictable instead of rigid.
For others, the notion of the stubborn fighters that refuse to accept defeat is an attraction. So, in the Horus Heresy the traitors lost their leader and much of their forces and got chased across the galaxy literally into hell, their support networks cut and homeworlds destroyed. Did they hole up and wait for the end? Did they give up? No! They tamed their warp-corrupted new homes, rebuilt and have continued the conflict for the past 10,000 years.
Further down the chart from the general notion of Chaos are the more specific groups. Each Legion has its own unique background, motivations, look and feel. Beyond the Legions are more recent traitors, renegades such as the Red Corsairs, the Purge and so on - or even a group of your own creation! Add to the mix the different 'religions' of Chaos, the four major powers (Khorne, Nurgle, Slaanesh, Tzeentch), a multitude of minor powers ripe for creation or fleshing out and the concept of 'undivided' Chaos that worships all. Some Legions or warbands are aligned to a specific god (like the World Eaters and Khorne), others have a mix of worshippers (such as the Black Legion where you can see Plague Marines of Nurgle fighting alongside Tzeentchian sorcerors), while others such as the Word Bearers pay homage to all without fully dedicating to any single patron and others still trust only in themselves, not the capricious whims of greedy gods.
These various elements can be used singly, or combined to create the force that appeals most to you. Maybe your army will be renegade White Scars that have turned to Nurgle, their bloated bodies carried across the battlefield by their bikes, plague spreading through their exhaust? Or perhaps Iron Warriors that are following the ways of Tzeentch to give them better insights into even more intricate battle plans and strategems?
Also, if you enjoy reading the 'fluff' of the 40K universe, Chaos has a lot to offer there, from inclusion in other stories as adversaries (such as the Space Wolf or Guant's Ghosts series) to books where they are the protagonists (such as Storm of Iron or the Horus Heresy series).
If you like your battles to have stories, Chaos can make things easy for you in that regard too. Obviously any battle with Space Marines, Inquisition forces or Imperial Guard is easy to explain! Throw the Eldar's hatred for Chaos into the mix while you're at it, especially the Harlequins. As for the others - Chaos slew their own brothers in pursuit of their goals...what makes you believe they'd think twice about getting rid of any Xenos that are in their way?
Chaos is blessed with lots of plastic and a wide variety of kits + fantasy Chaos + loyalist SM kits that can all either work together well or provide lots of bits. Lots of wild conversion potential - archaic and cobbled together technology, mutations and daemonic influences can all show their hand alone or in combination. There are also several generations of metal figures that can fit in as well if you're inclined to track them down - check out older Citadel catalogs at the Stuff of Legends (LINK) for examples.
Daemons are another area where you can go wild with 'counts as' and use a wide range of potential models, from GW's own daemon kits or other GW kits like fantasy dryads and beastmen, to figures from other miniature ranges (just be aware you might run into trouble using them at more restrictive events such as GW-run tournaments) or scratch-sculpted monstrosities.
The Chaos Space Marine army has a lot of similarities with regular Space Marines, so if you are familiar with loyalist marines the transition should be easy. Bolters are bolters, lascannons are lascannons, power fists are power fists. A unit of Raptors can fill the same basic role as a unit of assault marines as will a Predator and Chaos Predator.
In fact if you already have a Space Marine army, you have a ready-made renegade army waiting for you! You'll have to leave the Land Speeders, Attack Bikes, Crusaders and Whirlwinds at home but your tactical squads can serve as chaos marines, your Devastators as Havocs, your leaders as Lords/Sorcerors and so on, with Rhinos, Predators, Land Raiders and Dreadnoughts coming along with little difficulty. Doing so could be a great way to try out the Chaos list to see if you like it before investing in a new army. And who knows - you may like it so much that those loyalist marines wind up being permanently converted to renegades!
The Chaos army list has a lot to offer. Rock solid troops, a variety of support options and a good deal of customization available. The basic chaos marine comes with power armor plus a good stat line and is equipped with a trusty bolter, bolt pistol, close combat weapon as well as both frag and krak grenades. He's got good damage potential at range and in hand-to-hand combat and can potentially hurt any vehicle with AV12 or worse rear armor. About the only things in the game that he can't threaten are Wraithlords, Land Raiders and Monoliths. Not too shabby - and that's BEFORE you upgrade their weapons, make them cult troops or add a champion!
With the Chaos list you can easily go any number of ways. You can load up on terminators and obliterators and be a nightmare to anyone without lots of AP2 weapons. You can focus on bikes, raptors and transports, using mobility to win the day. You can focus on one god (making a Khornate or Nurgle army for example), you can ignore cults and icons altogether and focus on getting more basic troopers or anything in between. Players who want to enjoy the random nature can load up on Possessed, Dreadnoughts and Spawn, who might fail or succeed spectacularly. Theme players can find plenty to work with, and power players can make use of solid forces, flexible obliterators and devious abilities like the Lash of Slaanesh to challenge their tournament opponents.
Whatever part of the hobby appeals to you - the background, modeling, painting, playing the actual game - Chaos has a lot to offer.
But at what price?