Space Marine squads are six Terminators instead of five. A squad is typically one Sergeant, two types
of Heavy Weapon, and three Terminators with Storm Bolters. There are also non-standard squads
such as a squad all with Lightning Claws, Grey Knights, or the Legion of the Damned.
Heavy weapons options include most of the popular choices from Warhammer 40K,
including Assault Cannons, Cyclone Missile Launchers, and (although it's not a physical heavy weapon) having
a Librarian or Techmarine (with accompanying Tarantula Turret) in your ranks.
Librarians (and Grey Knights) are closer to the 2009 Space Hulk version, using a pool of 20 psi-points
as opposed to the old expansions' approach: all sorts of crazy psychic cards.
The system for line of sight and cover has always been a major sticking point in actual games. The
"I can see it!", "No, you can't!" argument should not be a major feature of any game. A system to smooth out
LoS has been added. Although it may be difficult to envision at the start, it eliminates the subjective
nature of LoS and is uncommon to need to use over-and-over in an actual game.
Tyranids have a more expansive army list. Genestealers still are the dominant populace of the board, but
rules are included for Termagaunts, Hormagaunts, Lichtors, Tyranid Warriors, Hive Guards, and the dreaded Hive
Tyrant. Maintained from old versions are rules for Hybrids, Patriarchs, and Brood Lords.
The rules are designed to work for larger player groups. A typical game might field near 7
players (four Marine Squads and three Bug players).