First, The CSM dex and the Ork dex are the bar to which other dex's are measured as to quality. I say this for 2 simple reasons. 1 they're balanced and 2. you can easily have a half dozen players all have completely different armies that are all competitive. Orks lose the recommendation just because they're labor-intensive. A new player may be overwhelmed when he asks what he'd need for a 1750 point army and the answer is 160 models. So Chaos it is then. Chaos has the ability to become the army you want it to be. First you can take Marine squads and add marks to give them a bit of a bonus and when you're ready there are cult troops. These are all troops, so you get a scoring unit that does what you like, be it survive, charge, stand and shoot or shoot then assault.
They're readily available, largely plastic and lack massive gaps in their model line that are becoming common in newer armies. New players don't always like the idea of reading about some cool unit only to discover there's no model (and sometimes no picture...) and they have to build one themselves.
Chaos introduces basically everything you need to know to play the game. They don't have any funny rules, but use most of the core rules you'll see. They have monstrous creatures, normal troops, fearless troops, vehicles, transports and just about anything else you'll need to know about. The only things missing are fast, open-topped or skimmer vehicles.
Chaos are more of a jack of all trades then most armies. Their basic troop can hold its own in CC, can bring special or heavy weapons and can take marks to specialize their mission further. Beyond that you have cult units. As a player gets to know himself as a gamer, he will discover what strategies he likes and what works for him. He can then expand with cultists that follow what he wants from the army. This is also an army that can grow with the player. With some armies when the time comes to expand, it's often more of the same thing. That's not a bad thing at all, but if a player has a budding force and wants to experiment, having so many choices gives new opportunities to try things out without completely re-building the army or changing its entire construction.
At the end of the day an army choice is something a gamer has to consider carefully. This is far from a cheap hobby, so its important to pick an army that becomes an investment. Just about any army can be licked up by a rookie and will draw them into the hobby if its right for them (sorry necrons, you still suck). Putting a few hundred bucks into some models means changing your mind completely isn't always an option. Having an army that can change with a player, do so many things and get them into the game is valuable so I usually will point someone to chaos first. Besides, Grandfather Nurgle always needs more children to spread his infectious love.