Its time to talk more about Tyranids, since that is on everyone's mind, and the codex is actually very complex. If you are a tyranid player, this codex is not a plug and play codex. Meaning that there are not any real "auto" include units or builds for this army. However, as we get further into learning more about how this codex is built, I think it will start to surprise many of us.
My first delving into the codex revealed that synapse is really the key to understanding the codex, and this was only partially true as I dig further into the codex. In fact, in this codex synapse is important, but however, understanding your units and Instinctive Behavior is just as important, and sometimes even more.
There are three types of Instinctive Behavior Lurk/Hunt/Feed, and creatures outside of synapse must take a leadership test at the beginning of each turn for each unit that is not in synapse that has this rule. The results are often catastrophic for units that fail these rolls, but there are ways to minimize the dangers of Instinctive Behavior in the tabletop.
Here is a link to the first article looking synapse creatures and the codex
http://natfka.blogspot.com/2014/01/lets-talk-tyranids.html
This is a look at Instinctive Behavior, because not understanding it will cause your armies to dissolve around you, as we have seen on battle report after battle report.
Genestealers
These units do not even have this rule. While most of the internet wrote these guys off immediately, they are the one unit that can operate completely outside of synapse with no worries at all about instinctive behavior. So these infiltrators have an immediate advantage that everyone has written off before they even delved this deep into the codex.
yes, lictors should of also been like this, but they aren't.
Instinctive Behavior Lurk
1-3 Survive: immediately fall back
4-5 Seek Cover: unit can only shoot if it is in area terrain, ignores difficult terrain, and cannot charge.
6 Stalk: like seek cover but add stealth
Units that have this rule
Death Leaper ldr 10
Termagaunts ldr 6
Lictor Brood ldr 10
Venomthrope Lrd 6
Obviously the units with lower leadership must remain synapse to operate fully. This is fine, since that only includes Termagaunts and Venomthropes which should be with your primary force anyhow where synapse is protected.
The other two can obviously operate outside of synapse with some degree of success as their leadership is high enough.
Instinctive Behavior Hunt
1-3 Burrow and Hide: unit goes to ground and fearless units instead prowl
4-5 Prowl: unit cannot run but must shoot at the closest unit. cannot assault either.
6 Destroy: same as prowl but gains preferred enemy
Units that have this rule
Hive Guard Brood ldr 7
Gargoyle Brood ldr 6
Harpy ldr 10 and fearless
Hive Crone ldr 10 and fearless
Biovore ldr 6
Exocrine ldr 7 fearless
Tyrannofex ldr 8 fearless
Its important to note that fearless units will get prowl most of the time when instinctive behavior occurs, which means the unit is shooting at the closest enemy unit. So while that can be a pain, its not as bad as the whole army disintegrating. The fearless units will be shooting anyway, and you can move to direct your targeting a little better since you already know which units are under instinctive behavior.
The non fearless units, like gargoyles, biovores and Hive Guard, must have synapse around them to operate effectively and efficiently since you do not want to burrow and hide.
Instinctive Behavior Feed
1-3 Cannibalistic Hunger: unit immediately suffers one hit per model in the unit at the units majority strength. units consisting of only one model instead devour. The hits have ap-
4-5 Devour: unit cannot shoot or run and must declare a charge against the closest viable enemy unit
6 Kill: same as devour but gains rage.
Units that have this rule
Tyrant Guard ldr 7
Old One Eye ldr 10 Single model only and alpha leader (all units within 12 use his ldr)
Hormagaunt ldr 6
Ripper ldr 5
Haruspex ldr 7 single model only
Pyrovore ldr 6
Ravener ldr 6
Red Terror ldr 8
Sky Slasher ldr 5
Carnifex ldr 7 can be single model or up to 3
Tyrgon ldr 8 single model only
Mawloc ldr 8 single model only
Single model units are not as effected by cannibalistic hunger, and thus can operate without being in synapse range better than the others. This puts a damper on Carnifex Broods with three models. Others like the hormagaunts suffer horribly under this rule, and must be kept within synapse range. Remember that again, you get to move your units before the assault phase, so you have some control on who the closest unit is.