As a relative newbie, here's my take on it:
Hordes: dunno - only ever played one game, and that was against Everblight. ToD kinda ran em at me in a "raaaargh" kinda way, and I kinda pulled a stealth maneovre thingy and befuddled those poor excuses for dragon worshippers into a hole in the ground.
Warmachine: Only played against a few factions, but have read all the faction books.
Cygnar: shooty, and kinda spelly. Balanced. They strike me as the ultramarines of Warmachine, but only cos theyre blue, most likely
Menoth: Burn!!!!!! Menoth love their fire (when applied to heretics, burning of). And chainsaws. Menoth are pretty damn hurty, up close. They also shut down spellcasters well.
Scyrah: Fancy doodah weapons and stuff. Never played against em. Their warjacks get energy shields. big girls' blouses that they are...
Khador: stomp. Hard as a steel bucket full of nails, with a cannon ball in the middle. Honking huge great warjacks, blokes with guns all over the place. The shooty, tough army. Think Dwarves and you wouldnt be far off, i reckon.
Mercs: bit of a mixed bag, really. Never played against em.
Cryx: The cool faction

Obviously, this is the one I know best. Cryx can be quite varied, actually. Your options are:
undead horde. Use a warcaster that can sac his own troops for power boosts, and supply him with large units of cannon fodder to chew on. Also useful for just throwing at the enemy.
Slinky: you can take lots of stealth units or ethereal units. (this means theyre either hard to shoot, or impossible to shoot, btw). Also potentially lots of magic stuff. I like this build.
Stinky: zombie pus horde. Think nurgle, but more explosive. I like this build, too.
Or, you can go elite, and take some really pretty damn dangerous H2H units and just go for the throat.
tbh, the big thing with WarmaHordes is your warcaster. Which warcaster you choose really does determine how your army plays. The options i listed for Cryx could probably be applied to otehr armies too, given the right caster. Thats what makes it such a fun game. You can buy a couple of casters, and have freedom to play around with totally different army builds.
Probably not the most useful post in the world, but hopefully I helped a wee bit. You could look here for some more detailed ideas.
http://battlecollege.wikispaces.com/