Had my first game of 40k 8th ed today against my younger brother. 2k points a side, my tyranid swarm facing off against his various Imperial forces. The game was a veritable bloodbath, ending in a decisive victory in his favour due mostly to the fact that in a kill points mission the swarm had far more units to lose. Regardless, I found the game every bit as enjoyable as previous editions and I feel the mechanics work far better than the preceding version of the game.
I also learned a number of things that will help me in future games.
1: Synapse is important! If you play bugs, do NOT let your little critters run around unattended. They will be gone in a flash.
2: Stratagems and Command Points can make or break your attack or defence. Load up on them and avoid using auxiliary detachments if possible.
3: The new AP system is a phenomenal addition to the game and a vast improvement over the AP system of 7th. Previously, tyranids found Custodes to be nigh unkillable. Today? My broodlord handled two of them on his own, and the remaining three were brought low by a tyranid prime and a zoanthrope respectively.
4: The lack of universal rules relating to unit types will have a huge impact on how models will be equipped. I know a lot of people with hive tyrants toting two sets of brainleech devourers. Previously these tyrants could hold their own in cc due to being monstrous creatures. Now? They have nothing to affect the armour saves of their enemies in combat.
5: Psykers are somewhat overrated. Smite isn't really scary and the buffs I've seen so far aren't all that groundbreaking. The psychic phase remains important but won't be as heavy for most armies out there. My brother managed a firm win despite having no psykers at all.
6: The ability to divide fire within units is fantastic, but also a thing to be aware of. When my warriors were peppering a tactical squad with devourer fire, my brother was not expecting the unit's venom cannon to round on his dreadnought. A similar nasty surprise awaited me next turn, involving a grav cannon and my carnifex...
7: Tervigons have gotten amazingly better.