Warhammer 40k -II- Tactics for the Tactics God

[Bryn]

Standard Grey Knights have, compared to regular Marines...
S6
WS 5
Storm bolters
Fearless
True Grit
The Aegis (an anti-psychic-power effect)
The Shrouding (they can only be shot if they're within 3d6*3")
A couple of rules which only affect Daemons.

They also have the chance to Deep Strike if taken as Fast Attack instead of Troops.

Advice for dealing with Genestealers:
Don't charge them. Genestealers, particularly with upgrades, are terrifying in close combat, and against all but some Eldar they will strike first and they will decimate your unit before you can do a thing against them (especially with defenders piling in under 5e).

Get into terrain - this will let you get some hits in when they charge you, hopefully killing enough to save your unit. Thankfully, Genestealers don't get any sort of frag-grenade equivalent as far as I can tell.

Templates, especially AP4 templates like the Inferno Cannon and Heavy Flamer, are your best friends. Most armies can't get Hellhounds, but multiple heavy flamers work just as well. If the Stealers are foolish enough to run across open ground don't hesitate to drop blast markers on them.

Edit: I should add that I'm speaking here as an Imperial Guard player. It might be that you have better odds as an army with lots of close combat weapons.

The Genestealers will be hitting on a 3+ all the time - Feeder Tendrils costs an entire 1 point to add its effect (favoured enemy) to the entire brood. Though Rending has been nerfed, it's still nasty - no armour in the game will protect you against every six they roll to wound. If the Stealer player spends 4 points (not per model, just 4 points) they'll also be re-rolling to wound.

Perhaps not as bad as all power weapons (), but still very good and definitely not something you want to be in combat with.