The Rock Paper Scissors of Warmachine
While Warmachine can never be as balanced as Rock Paper Scissors, there are many elements in the game that work similarly. For example, a Khador Juggernaut is good at killing warjacks/warbeasts. Now that's more like Rock beats Rock, except that the second rock isn't always as good at fighting back.
Warmachine is far more complicated than Rock Paper Scissors. It's more like Rock Paper Scissors Lizard Spock. The point I'm trying to make here is there are several things you need to prepare for when building lists.
Rock beats Rock?
The two most important things to prepare for when building a list (this should be obvious) is a way to kill heavy warjacks/warbeasts, hopefully in one turn, and a way to kill a good amount of infantry. These things can come in a wide variety of solutions. For example, a unit of Bane Thralls are fully capable of killing a warjack while they might struggle at killing a lot of models in a unit. Thunderhead might be capable of clearing a large area of infantry while having trouble taking down a warjack. On the other hand, the more traditional solution to killing warjacks is with warjacks, and infantry killing other infantry.
Lizard poisons Spock
Here is where a lot of subtleties to list building come in. One of the reasons Warmachine is such a balanced game is that everything has it's Paper and it's Scissors. In other words, there is always something that your opponent can bring that will counter some cool things in your army. A good example of this is Lich Lord Asphyxious (eGaspy). There are many, many posts in the PP Forums about this warcaster and how he's so powerful. However, if his opponent is Captain Victoria Haley (pHaley), he will have a very hard time being effective at all. Furthermore, pHaley's spell, Temporal Barrier essentially means that eGaspy doesn't have a feat. The point I'm trying to make is that everything has a counter.
There are several things after warjacks and infantry that players can plan for. These are different, however, because some of these things depend on local meta (essentially what other players tend to bring). A short, but by no means exhaustive, list of these things would be: stealth infantry, high ARM infantry, heavy infantry (multi-wound infantry eg. Man-o-wars), high DEF infantry, buffing (beneficial) upkeeps, debuffing (harmful) upkeeps, or eEiryss. There are counters to all of these things, and most of the time each faction has access to each counter.
Putting it to Practice
I'll just go ahead and share a list I have been working on and give some reasons for some of the selections I have made.
Wolves of Orboros
Wolf Lord Morraig
First of all, I chose pKreuger for his ability to shred through large amounts of infantry, both on feat turn and not. I thought that because of his infantry killing, I could afford to go a little heavier on the beast side. I chose the Feral, because he's a solid beatstick, with MAT 7, and he's a great Lightning Tendrils target (gives Reach and Electro Leap). I took the Gorax for his animus and his ability to kill a light given the need. I opted for two Woldwardens for maximum Geomancy. They really only have two spells they can cast, but those two spells are amazing. Instead of needing pKreuger to be casting Chain Lightning all the time, the Wolds can do it, and leave Kreuger with Fury to cast Skyborne and Deflection. That's it for the battlegroup.
The Druids are a meta choice, in my opinion. What I mean by that is this: their usefulness depends on what your opponent will likely bring. If your opponent has a lot of offensive spells or elemental attacks (read: Cryx or Legion, respectively), the Druids will be very effective. However, against some other choices, they might not be as effective for their high point cost. Some people would rather just have another offensive unit. I chose the Wolves because they are quick, cheap, and also amazing Lightning Tendrils targets. While they won't benefit from Reach, they sure love the Electro Leap and the Electrical Immunity. They become good targets for starting Chain Lightning if you're fighting stealth units. Not to mention the fact that they can tear a unit to shreds with MAT 8 on a charge and an auto-hitting Electro Leap attack. Next is Morraig, who hits like a truck. He really likes to charge non-Reach warjacks, rip them in half, and laugh as they can't kill him next turn. Finally we have the Blackclad, who is another good target to Chain Lightning from. He is also great for his magical spray and Hunters Mark.
So there you have it. Just a crazy man's ramblings in the middle of the night.