Over the course of 5th edition, various meta-defining army lists appeared. These ranged from the well known Grey Knights paladin armies marshaled by Draigo, through the Imperial Guard "leaf blower" army lists, ork biker Nobz death stars, and others in between. Each of these ideas came about from a single codex (in general), although some did feature some generic traits (e.g. a very tough core unit; plentiful firepower, etc.). Most players at tournaments will have encountered these kinds of list at some stage in some form or other. Arguably the strongest meta-idea in 5th is a full-on mechanized list. Plenty of players packed multiple melta-guns (or similar) to deal with these eventualities.
I think that in 6th edition, the meta-defining army lists will no longer come from solely a singular codex. Much like the mech-lists of 5th, I think that various concepts will come to the fore in the style that army lists are defined. So, I'm going to make a prediction here: I think the next major meta-game will be "flying".
Well, the recent focus on mechanized flyers, the promised chaos dragon flyer (and others) all make me think that almost every gamer out there is going to be opting to select at least some flyers. This will mean that all army lists are going to require something to counter this style, or join in with this style.
For me, I'm toying with the idea of 5+ daemon princes or flying daemons in my chaos lists. Consider this: daemons can field 2 HQs with flying (bloodthirster / lord of change) coupled with 3 Heavy support flying daemon princes. Add in some chaos space marine allies for an extra HQ flying daemon prince, and you can see my point here. Granted, this isn't the strongest, most points efficient list that could be built (I'm playing daemons and chaos marines, remember!), but I feel that multiple flyers (whether monstrous or mechanical) will be the way forward.
That said, I think that we're going to see an increase in weapons that have skyfire (and/or) interceptor special rules as well. The reliable quad gun comes as an upgrade to the Imperial Bastion or Aegis Defence Line and I therefore think that we're going to see a number of players go down that route as well. i.e. rather than radically rebuilding lists to go with multiple flying options, simply tweak existing armies to ensure an adequate spread of skyfire weapons. For (arguably) a low tier codex like daemons, purchasing a fortification with a quad gun suddenly looks very tempting.
So, there's my prediction! Happy to be proven wrong. Maybe fortifications are going to be the new meta to beat? But I'm also interested in what other folks reckon the meta-defining feature of 6th might end up being?