Imperial Guard:
Boost: Valkyries/Vendettas, Hydras, Manticores (new), Veterans, Chimeras ((arguably) underused).
Nerf: Leman Russes, Basilisks.
Tyranids:
Boost: Trygons, Hive Guard, Tervigons, Tyrannofices (new); Devourer Gaunts, non-winged Tyrants (underused).
Nerf: Pyrovores (new); pretty much everything else, most notably Carnifices and Spinegaunts.
Gray Knights:
Boost: Henchmen, Paladins, Storm Ravens, Purifiers, Draigo, Crowe (new); Coteaz, (Rifleman) Dreads (underused).
Nerf: Terminators, mixed-weapon PA Knights, Land Raiders.
Necrons:
Boost: Barges, Scythes, Crypteks, Imotekh et al (new); Scarabs, Spiders (underused), Wraiths (used).
Nerf: Guard (new); Monoliths, C'Tan.
Space Marines and clones:
Boost: Thunder Wolves, Sanguinary Guard (as of 6th), Rifleman Dreads, Storm Eagle, Sternguard, LR Crusaders, Vulkan et al (new); Typhoon Speeders/MM+HF Speeders, Librarians, Razorbacks (incl. new Plasmaback), TH Terminators, Missile Launchers (underused)...
Nerf: Land Raiders (new Crusader better), Chaplains, Terminators (Cyclone/AssC), Claw Termies, PF Sergeants...