Just from the reading I've done, yes I'm in the haven't got to play yet catagorie, it's been hard to figure out some of what GW is trying to do. Ive been a bit confused since the Necron codex came out about this formation stuff. I've always just used a basic building structure for all the years I've played this game and now, brain pains. So, if I want to play my Iyanden army, I can take the formation and it is still a bound army? Or do I have to take the guardian thing to do this? I love Saim Hann so that formation works for me. Can you build an army with just the old structure or do we have to use the one in the book? I'm so lost with that.So instead of giving helpful examples and info, I find myself lost in the abyss of building structures first, once this is resolved and I've gotten some play time in, I hope I get back to my old helpful self.
To hopefully clear this up for you, there are a few ways to build a "legal" list these days.
Combined Arms Detachment (CAD) - This is your standard force org chart with 1HQ, 2x Troops minimum with the 3/3/3 elite/FA/heavy, Lords of War, and Fortifications as your Auxiliary choices. Troops units get an ability that gives them priority on objectives over all other models that don't have it (everything is scoring now).
Detachments - These are usually altered versions of the above. They will usually require a certain force org slot in addition to the normal ones (example - 1HQ, 2x Troops, 1x Elite minimum). Troops generally lose the special rule for priority over objectives but the army will gain some other special ability and/or bonuses for building your army this way. Using the above example, you might have to take 1 HQ, 2x Troops, 1x elite as a minimum, but then get access to 5 more elite choices (so 6 in total) and your whole army gets +1 initiative on the charge. This is a pretty tough choice as the extra elite slots, and extra initiative on the charge could outweigh having "better" scoring units.
Formations - Formations are still detachments, just special. Formations usually require very specific units to be taken instead of a variation of the normal force org chart. Formations always have special rules associated with taking the specific units of the formation. So the Aspect host, or Avenger Shrine are an example of a formation.
One thing of note - options in each CAD/Detachment/formation do not count towards other CAD/Detachment/Formation options. Troop choices from a CAD, do not count towards the troop requirement for a Detachment or any combination there of (including formations). They are all their own list of requirements and units do not cross over.
Taking a CAD, is a "legal" list as it has always been. Taking an army made of just a special detachment, is still a "legal" list. You can also make a list of nothing but formations, and still be a "legal" list. You can also mix and match all of these, and STILL have a "legal" list. I have been putting "legal" in quotations because "legal" is relative. Technically speaking, an unbound list consisting of whatever models you want with no regard for any structure is technically still "legal" to field according to the BRB.
In regards to the new way GW is doing it - you have the the Warhosts for example. The Windrider and Guardian hosts are a formation that requires specific units. This group of units get special rules. They get one army wide bonus, and then the formation themselves get their own bonus. You also have to include at least 1-12 other formations along side one of the warhosts. So you would attach, for example, an Aspect host. The aspect host gets their own formation bonus, as well as the army wide bonus (I.E the better run movement). Building this way can net quite a few special rules, but is more restrictive in how you can build. Some times it makes for a more fluffy army to play with/against as you will have a nice mix of units. However a lot of times, it is GW trying to force you into using (thus buying) models not normally used.
So yes, you can run nothing but the Wraith host if you wanted to. You could still create a normal CAD, using the units in their respective slots - however this is more difficult due to the changes of the spiritseer. However, nothing technically is stopping you from just running all of your wraiths if you wanted. You could also make a list of nothing but the Aspect Host (fitting 2-3 in say a 1500 point game) and be a perfectly legal, bound list.