Army Construction Concepts

Here are a few things that I consider to be among the most important principles in army list design.

Overall Performance 

Often are eyes are bigger than our heads when it comes to a new army we have started or a list concept that has intrigued us. It's easy to say something like "Against SW this list is sick!". Is it still sick going 2nd? How does it do against IG, BA and GK? So what are it's match ups like? Does it struggle to massacre or even just get minor wins against many kinds of lists? This is why I didn't like Deathwing after just 5-7 games of playtesting.

Can the army start in reserves and play out the game reasonably well? This can be really important when playing against an all shooting army like IG. It is why I wouldn't play an all shooting army that wasn't GK(only because of Coteaz and Shrouding).

If you are playing in a MoV tournament you need to play an army that can get max points frequently. All it takes is one draw and you are praying to Table your last opponent AND praying the guys at the first table draw. Sounds unlikely to me. Here is an idea, assume it is the last round and you are going second vs IG. If you need to get a massacre to win the tournament set up anyways and roll to seize. Sure rolling a 6 is very unlikely to happen. Yet it is more likely that you will get the max points this way then going second and hoping for the best. I suppose this idea has more to it on a table with sparse terrain.

Versatile Offense

Do your units threaten tanks and infantry alike? If you have 12 units and 6 are great against Infantry but poor at killing or suppressing tanks and the other 6 are the opposite you might be able to do better. I won't take an army to a major tournament that doesn't consist almost completely of units with the ability to do both.

A good example is 5 BA Marines with Melta/Infernus Pistol in a Las/Plas Razorback. The team of 2 units is good at shooting MEQ and vehicles. Not perfect but somewhat well rounded. It doesn't do much against fearless mobs of Orks but if you are against non-MEQ infantry you can plow through multiple squads at once. The odd tank shock(or 20) can be a great offensive tool. LD 9 fails about 15% of the time. If you tank shock enough you can get some sweet freebies.

Mobility

 Deep Striking, Outflank, Infiltrate, Fast Vehicles, Bikes, Jumpers, Shunters, Flip Belts. All of these things can be pretty awesome, much better than just foot slogging. If your list doesn't have much of this you should take a look at the codex. Having to go second is much better with a mobile army. You can potentially dictate the entire pace of the game. If you have to take a lot of terrain checks in a game the chance of catastrophic failure gets higher and higher. Nothing like losing a game because not one but two squads just couldn't slog through the muck. Not a good way to go out.

Resilience

Units have got to be tough(or so numerous it just doesn't matter). Plague Marines and Obliterators used to be the perfect example. I guess these days things are easier to shoot down but you can use LoS blocking terrain and vehicle wrecks with special rules like Stealth and Feel no Pain to make it nearly impossible to scour your army off the table with just shooting.

I would generally avoid anything open topped unless you are a really great player. I know I am not good enough to win an event with such a big disadvantage for my opponents to exploit so I just say no. It's why the Wagons are closed top. Letting Melta Guns get +2 on the damage chart is a recipe for disaster. Any thing that can be exploited should be studiously avoided.