Hey Everyone! WARNING: There is a LOT to read in this post!
One of the interesting developments that people are starting to take advantage of in 6th edition is the change to how Special Rules (formally known as USRs) are worded, and how they interact within units. Most importantly, certain rules (listed below) simply require one or more models to have it in order for the entire unit to benefit. With the inclusion of Allies, armies are able to take even more advantage of this.
Below, I've compiled a list of all of the special rules that have this wording, with a little description of how this can be used.
First, these rules have no models currently:
These rules are only found/usable in armies which are not Battle Brothers for Dark Eldar:
And They Shall Know No Fear
Slow and Purposeful
And here are the rules that are found in either Dark Eldar or Eldar units:
Acute Senses - When coming in randomly on a board edge, you can reroll the d6. Maugan Ra (Eldar) has this SR. Unfortunately, the only ways for him to join an Outflanking unit is through the Personal Trait "Master of Manuever" or in a unit with Scouts. Not something worth banking on, and even if it does happen, Maugan Ra is better placed somewhere else.
Fearless - You're... Fearless. Vect and Drazhar have it in our codex, Pheonix Lords and Wraithguard have it in Eldar, and most units in our army CAN get it if they get enough pain tokens. This is a HUGE improvement. We can give Beasts Fearless right away, which is perfect for a LD 8 assault unit. There's only 1 model that has Fearless right away that can keep up witht eh Beasts, and that's Baharroth. I know, that model would look pretty nice running along the board.
This also makes all the Pheonix Lords a good choice for an allied HQ if you don't want the Psykers, and makes Vect and Drazhar pretty beastly as well.
Hit & Run - Leave combat as you please. The Baron, Hellions, Baharroth, Shining Spears, Warp Spiders, and both Harlequins get this ability. We all know about The Baron already, and giving Beasts/Grotesques Hit & Run has some great power. Shining Spears, Warp Spiders, Harlequins, and Hellions will confer this to an IC. Not really too useful, but it can give you an extra 3d6 every once in awhile. And lets not forget Baharroth. Fearless and Hit & Run being conferred to Beasts without slowing them down? Althought, it is REALLY expensive.
Move Through Cover - ICs no longer have this as default. Striking Scorpions, Rangers/Pathfinders, and Mandrakes have this SR, meaning each IC joining them can benefit. This is particularly useful for a Haemonculus attached to the Mandrakes. Otherwise, this is a pretty minor bonus.
Night Vision - We have it everywhere, and Eldar have it nowhere. Plug an IC in a unit of Reapers or Rangers for some extra damage during Night Fight, or put Maugan Ra in a unit so he can still use his weapon unhindered.
Preferred Enemy - Vect has this rule, and now that it benefits all, this can be huge. I think the best move is putting Vect in a poison shooting squad to make them pretty dangerous on the move with rerolls of 1's to shoot and assaults, hits and wounds. And with Vect giving them a nice combat boost, no one will really get close to that unit. Another option would be Rangers/Pathfinders, as they can reroll for more chances at AP1/Rending/Precision Shots, or Wracks/Haemonculi with Hex Rifles for similar reasons. The Wracks/Haemonculi can also be a decent combat unit to accompany him.
Scouts - Pathfinders (upgraded Rangers) have this nifty ability, which can help bring your ICs up a bit closer to the enemy if needed. However, You can also Outflank with this unit, and as far as I can tell that ALSO is conferred onto an IC.
Stealth/Shrouded - In a world with lower cover saves, Stealth is King. The Baron, Mandrakes, Rangers/Pathfinders, and Harlequins with Shadowseers have this SR (Harlequins have both), and any unit with a PGL will have Stealth up close. This really only gives a major benefit with the Baron though, and we all have known this since the codex was released thanks to his wording. Even then, a late 5th edition FAQ reworded it to work as it currently does for everyone. Nothing special here.
Skilled Rider - Reaver Jetbikes and Shining Spears have this ability, but unfortunately no one really benefits much from it. Jink isn't conferred to any ICs joining them (other than Eldar ICs on bikes), so that bonus is lost. The Dangerous Terrain ignoring is nice, but again only benefits a handful of ICs, and nothing majorly (Eldar ICs again, or the Baron, who already gets a reroll).
Tank Hunters - Fire Dragons have this ability, as does Fuegan. Unfortunately, because of an FAQ, Pheonix Lords can only confer exarch powers to their Aspect. So, the only real benefit we can get from this is adding someone with a Blaster, Blast Pistol, or maybe even a Gunslinger to a unit of Fire Dragons (with Exarch). We can do this with an Archon to give them some nice combat utility, but unless we go witht he blaster, we're going to have some relatively short threat range, since they wouldn't be able to ride in a vehicle together. If only Fuegan could give this to Trueborn. Infantry based Void Lances would've been fun!
Overall, I see the valuable Special Rules as Fearless, Hit & Run, Preferred Enemy, Stealth/Shrouded, and Tank Hunters, with Fearless and Hit & Run at the top, and Preferred Enemy towards the bottom - for us that is, as we only have one model with Preferred Enemy.
I think taking advantage of this change in the rules (as well as understanding and being able to adapt to opponents) is going to be pretty important as we all adapt to 6th edition. I think the Fearless change is big all around, as most armies have at least one Fearless IC.