So with NOVA over and some feedback from others going on, let's look at what's been kicking around my head lately in terms of list constructs for 6th edition. This is by no means the be all and end all, it's just things I've noticed throughout practice games.
1) Mech/Foot relationship
NOVA put a pretty hefty emphasis on what 6th edition has done to vehicles in terms of not scoring and not denying with table quarters. Despite this, a lot of the top armies had vehicles in them, just the top 2 didn't. So everyone unwind your knickers here - yes I think table quarters need to be re-visited but no vehicles aren't dead. What is dead is the vehicle spam where it takes up over half your army or even a significant chunk in terms of points. Even in normal objective missions, that's too many points which are essentially useless when it comes to achieving/denying end-game mission parameters.
This isn't to say vehicles are bad. Far from it, but you can't spend as many of your points on them as you used to which means foot units are going to see more use. The good mech is going to be stuff which is efficient and causes lots of damage. I.e. Manticores, Psyfledreads, flyers, etc. or vehicles which are cheap and bring all the tools of their trade to the midfield such as Rhinos, Chimeras, Psybacks, etc. Spamming 6+ LasPlas or Passbacks, etc. is probably a thing of the past though they aren't completely dead. Vehicles with higher AVs and ranged weapons are likewise still a solid concept - Glancing hits are no longer suppression tools and higher AVs limit the ability of weapons to do Hull Point damage.
Regardless, vehicle saturation is less likely in terms of overall points used though you still might see quite a lot of vehicle hulls running around. These extra points are going to go into larger or more infantry units and thus a trend of more infantry is likely to be seen.
What's really important here as well, is both unit types help each other. Any infantry model with a grenade is a scary prospect to a vehicle. Even mass S4 attacks will reliably strip a Vehicle of Hull Points in short order unless it has AV11 or better on the rear. Combat is a place vehicles don't want to be and infantry can help protect them from opposing combats whilst also giving vehicles cover thanks to the 25% rule. This means vehicles can provide the mobility and cheap/efficient firepower you want whilst also bringing in the LoS blocking and Flat-out options infantry really want later in the game when you need to keep them alive. It's a mutually beneficial relationship where both unit types can help each other out throughout the course of a game.
2. Flyers
Spamming them sucks. This seemed to be pretty widely accepted before hand but taking any more than three, MAYBE four, is just a bad idea. Not only do they suffer like vehicles above but they don't bring the utility that a bawks on the ground does and if you don't have hover, you're losing firepower for most of the game in some capacity. Yes they are a fine option but they aren't something you need or you need a lot of. There are a lots of ways to counter them without actually shooting at them. Flyers without hover are also far less scary due to their limited mobility and flyers with Hover become a lot easier to shoot down if they start hovering.
3. MSU (multiple small units)
Part and parcel of the vehicle aspect, mass MSU appears to be, for all intents and purposes, dead. That's not to say MSU aspects within an army don't work but if your scoring is centered around only MSU units, well expect to lose. Firstly, you'll likely be spending more points on vehicles. Again, not an issue if done right and the balance is found (i.e. 8 Rhinos is a paltry 280 points, I can live with that. 8 Psybacks is 400 - still workable. But 8 Passbacks or Wave Serpents? That's 700+ points...) but if that's the entirety of your army, the improvement in shooting, particularly anti-infantry fire, is going to see them drop pretty quickly. Changes to Rapid-fire weapons and cover (and thus an increase in plasma weaponry) means 5-man Marines die pretty quickly now BUT anytime you get left with just a couple of models, they are very easy to hide with the amount of Wrecks strewn across the table.
That's relying a bit on luck though and thus having a couple larger units out there to suffer the improved shooting is going to be important to ensuring units make it to the end of the game to score. MSU is a great addition to an army to give one more options, firepower and flexibility but by itself is going to struggle to maintain momentum in 6th edition.
4. The Relationships between all of these
What's really important is balancing all these factors together. MSU maximised might not be as good but the core concepts of what MSU does (flexibility + more firepower) are still key. If you can get six large units in, that's better than four large units, etc. Vehicles and mech transports are still solid options just not as spammable. Taking something like backfield support vehicles such as Predators, Dreadnoughts, etc. along with CHEAP and EFFICIENT midfield vehicles/transports gives you the ranged firepower and durability you need along with all the utility options a moving metal bawks gives you in midfield WHILST adding to your firepower and army.
In the end it goes like this. You need scoring infantry to win games. You need infantry to stop your opponent from winning games if your firepower fails. Everything else needs to support this so if you're finding games where you cannot deny your opponents their objectives whilst maintaining yours, you're probably looking at too little infantry and/or firepower and need to tweak that balance in the other direction. If you find you're being out-manuvered and having difficulty keeping your infantry alive, you're probably looking at too much infantry and not enough support, etc. (though pure foot lists are more likely to see use in 6th).
Conclusion
I think the major conclusion I've drawn so far, and it appears many others have as well, is Hybrid and balance is where it's at. There will always be extreme builds which find themselves doing well, but I imagine armies which are mixing large and small units and foot and vehicle units are going to be doing the best overall. You're getting the best of each world whilst minimising the inherent weaknesses of each.
In the end, Hybrid is really the name of the game so far. And yes, this is still mostly speculation and thinking - but it's been/being honed by playing experience. We could see this evolve a lot or a little over the coming months.
