Had my first few games of 8th ed. with the GSC (previously having tried out my orks, only to hit a brick wall of Deathwatch frag cannons, Blood Angels getting their teeth kicked in by Scions, and AdMeck getting mauled by Tyrannids) Here's my observations after a 1500 pt game against Spehss Mehreens.
1) Purestrain genestealers. Good God almighty. (in a good way) These were the MVPs, 10 of them tearing apart a Venerable Dreadnaught, a Land Speeder, and a Space Marine Captain during their rampage.
2) Hybrid Metamorphs. Good God almighty. (in a bad way) They pretty much seem to be more expensive Acolytes without the special weapons.
3) Acolytes... I'm sure its been said, THIS DRILL IS THE DRILL THAT WILL PIERCE THE HEAVENS!!!
4) A 5+ save seems to make a huge difference compared to the ork's 6+, especially with how common AP is.
5) Iconward + 2 blobs of Neophytes = a lot of survivable dakka.
6) GSC pure is lacking Heavy Support. The answer? LEMAN RUSSES!