Genestealer Cults were largely defined in their play style through their very cool and characterful Cult Ambush special rule. That is still true in the new Warhammer 40,000 as well. Cult Ambush allows you to place Genestealer Cults units into reserves and then enter the game through a variety of means determined by a D6 chart. These range in effect from coming on to a table edge to being able to appear anywhere on the table more than 9 away from any enemy units but being able to move as well, allowing you to get extremely close to enemy units to shoot and then charge them as well. For units like Genestealers or Aberrants armed with power picks and rending claws, this can be punishing!
Speaking of Genestealers, if you thought they were deadly in the Tyranid army, they are even more flexible with Genestealer Cults due to the Cult Ambush rule as stated above. Combined with the Patriarch, they hit just as hard as the Broodlord and Genestealers do with Tyranids, but with Genestealer Cults, the Patriarch also allows nearby units to ignore morale, and he benefits from the Unquestioning Loyalty special rule which all Genestealer Cults Characters posses. This allows them to avoid taking a wound on a 4+ as a minion sacrifices its life and jumps in front of the attack. This makes these Characters extremely durable and even helps them survive things like Sniper fire.
Another nasty trick the Genestealer Cults have is their Mass Hypnosis psychic power, which prevents an enemy unit from firing Overwatch, makes them strike last in combat (even if they charged) AND gives them -1 to hit. Ouch! That leaves an enemy unit in a very compromised position as the minions of the Four-armed Emperor emerge from ambush and then run them through in melee.
One of my favourite aspects of the Genestealer Cults, though, is their ability to form their army using both their own faction, Tyranids and Astra Militarum, too. Huzzah! Due to the way that allies worked in the previous edition, making an army that mixed Genestealer Cult units and Astra Militarum wasnt always straightforward. These are, of course, not real Astra Militarum, but devoted followers of the Cult who have turned from the Emperors light and betrayed their fellow man. The way it works in the new Warhammer 40,000 is really fun. You can take one Astra Militarum Detachment for each Genestealer Cults Detachment in your Battle-forged army. They each need to be entirely comprised of units with their respective keyword, but this allows incredible amounts of diversity in your army that is also quite characterful.
But you wont always need to sneak into the Imperial motor pool to hijack their vehicles; the Genestealer Cults have access to some fantastic mechanised war machines of their own. The Goliath Rockgrinder is one of my favourites.
For armament, I prefer the Clearance Incinerator, as it automatically hits its target D6 times up to 12 out and, of course, you have to take the Cache of Demolition Charges! While you can only fire this weapon if a unit is embarked upon it (as they need to chuck them out!) and it is short ranged at 6, boy does it pack a punch! D6 shots, Strength 8, AP -3 and D3 damage per hit. Thats no joke. If anything survives that, you can ram them with the Drilldozer in melee for a possible 6+D3 Strength 8 attacks with an AP of -2 and D3 damage per hit.