Join Date: Feb 2012
Location: Southern GA, USA
I can only really tell you about the Legions that I've seen in my own local store as far as play value is concerned, but perhaps it will help. For fluff, you're really better off reading the HH books themselves to get a good feel for that, as it would be a very large chunk of TL;DR if someone really tried to do it all here.
First off, I play Salamanders as my dominant 40K army. The way I play them seems to work against most armies given that I've only ever lost to the Sons of Horus, and that was only one time. I use Breacher Squads to stunning effect, as they are both good at weathering fire power as they foot schlog down the table as well as good at dealing death to tanks (since you ALWAYS take melta bombs when given the option to). I also use Apothecary Detachments to stunning effect in my Legion, making my units less and less squishy as we go. Firedrakes are my mainstay as far as CC wrecking balls are concerned. Two wound terminators with a 3++ invul is more broken than anything else in my arsenal and I like it that way... And I cheat a tad and give them a Primus Medicae so they have feel no pain even though you can't normally stick an apothecary in their unit because... yeah. Beyond that, I use either Contemptor/Mortis Dreads, Sicaran Battle Tanks and Tank Destroyers, or my Deredeo Dreadnaught to deal pain and punishment as my army and mood requires and dictates. And once in a blue moon I throw tarrantula gun turrets down range because I'm evil like that.
I have a friend that plays Alpha Legion, and they are all about the shenanigans. If you aren't using Moritats, Destroyer Squads, and all the rad and phosphex you can get your evil little hands on, you aren't doing it right. My friend always gives me a run for my money with his Alpha Legion, but generally I can out maneuver him even when he does steal my initiative. It's a worth while army for anything that like to be subversive and evil in battle.
The Sons of Horus player I game with is all about brute strength and a whole lot of bodies, more so than you would think for them. He doesn't use a lot of heavy tanks, but he does sneak in a damnable flyer once in a while which can wreak face when you don't prepare for such things in your list. But I have noticed that they are a much more CC oriented army than some of the others out there.
We also have a Death Guard player, but I don't think he knows how to use them because I don't think he's won once with them... So perhaps going into details about them will only be counter productive at this point.
World Eaters, I know, are all about throwing bodies at a problem and hoping they get overwhelmed. Red Butchers, Angron, and Kharn teach you that lesson just in their state lines. Throw in their other special unit whose name escapes me right now, and something to drop them dead center into enemy lines (I recommend the Kharybdis Drop Claw just for space's sake) and you're cutting a bloody swath through all but the most heavily armored armies in the mix.
Night Lords have only ever done one thing in the shop, run away... which seems counter intuitive except that the player is a bit of a wuss and doesn't play as well as he paints and collects. They look awesome, but the boy just don't want to fight and that's all they're good for is fighting. I would say they are another CC army but I can't be sure of that since his face got wrecked by my Firedrakes in Zone Mortalis and he never played them again...
Hopefully that helps you a little in getting a feel for how some people use the armies. But I would agree with Sworn Radical, a little research may be in order for you to better explain things to those you want to convert to the resin crack hobby.