Reactor Blowouts
Every once in a while the reactor goes critical, changing the ambient sound and shortly after, instantly killing any living thing that isn't hiding.
To avoid it you must enter a protected indoors area where you can't hear the reactor's ragged "breath" anymore.
NPCs and enemies have various reactions to these reactor blowouts, like the Zerg who will abandon fights and burrow out of sight until the danger is gone.

Interiors and Taking Shelter
The main purpose of interiors is avoiding reactor blowouts, but not all interiors are safe, some are exposed to the reactor through gaping holes through which you can hear the reactor.
A safe interior will always play its own ambient.
Some are entered by Using an entrance while others can be simply walked into.
You can find random items, enemies and rarely vehicles on your first visit in them, after a very long time all of the interiors will refresh their contents.

Don't leave behind items or vehicles in randomized interiors as they will be lost.

Randomization
Most interiors are randomized when the map begins, which means:
- safe/unsafe shelters are rearranged each time, without breaking survivability
- merchants and computer terminals can be located in different places
- items and vehicles and many other objects are randomized, some important tools might be found when searching interiors or they might need to be traded from merchants

Interface
You can either hotkey the Starport and train units from it, or more ideally use the EUD hotkeys to control it without needing to select it.
a) Wraith: Use (press 1)
Generic action, enter interiors that have entrances, disarm spidermines, pick up items, talk to NPCs, etc.

b) Dropship: Look (press 2)
Provides hints on how to progress in some areas.

c) Science Vessel: Use Tool (press 3)
Lets you use any special tools you've acquired.

d) Battlecruiser: Heal (press 4)
Uses a MedPack if you're not driving a vehicle and a Repair Pack if you are.

e) Valkyrie: Energy (press 5)
Uses an EnergyPack.

You can also use a Dark Archon to examine notes later into the game, use his Mind Control on the gun turrets.

Items
MedPack - restore health of your footsoldier
RepairPack - restore health of the vehicle you're riding in
EnergyPack - restore energy
Spidermine - can be placed once you have the Minelayer tool

Hack Tool - can hack nearby gun turrets to gain control of them, when hacking blue text is scrolling down the screen and you must stay still or the hack will be cancelled
Effraction Tool - enter and exit vehicles with it, vehicles must be neutral
Minelayer Tool - place spidermines you've disarmed or acquired from sellers, blows up both enemies and some debris barriers

Upgrades
You can use upgrade points (gas) to boost your weapons and armor with the buildings in the bottom right corner.
Upgrade points are acquired when you decode one of the Echelon Disks, found in semi-secret places as pickups, and can be bought from the Engineer at home base.

Basic Guide
Move carefully from shelter to shelter, gather all items you can find and sell the ones you don't use at the Bar or at the random merchants, generally avoid fights unless cornered, save often.
Check the large map image for hints.

EUDs are used to control all vital functions of the map with keypresses.
1,2,3,4,5 control the old functions of the Starport, used to interact with items, converse with NPCs etc.
ZXCV are new quick function keys that bypass submenus and allow you to:
Z - enter or exit a vehicle
X - place a spidermine
C - hack a nearby object
V - check your inventory

All classes have received enhancements to differentiate them and adapt their abilities to the map they are inhabiting.
Natural energy regeneration has been removed so that Energy Packs are the only source of energy.
Vehicles can comfortably move up and down staircases.
New areas, units and doodads have been added to the map.

— Marine
- stimpack gives 16% more movement speed than before, graphics overlay while stim is on
— Firebat
- stimpack gives 16% less movement speed than before but also absorbs up to 20 damage, stim graphics overlay
— Medic
- new weapon: Flash Cannon (no damage or aggro, stuns organics, can still destroy very weak units like Barrels and Infested Terrans, weapon upgrades increase duration)
- new AI and ordering behavior to closely match normal offensive units
- new ability: Aid (heals and removes status effects from either self or an ally)
- new ability: Cover (smokescreen that causes 75% of ranged attacks in its area to miss)
— Ghost
- new ability: Magna Pulse (channels to stun a unit of any type, Ghost can still move but cannot attack or use abilities, unit remains stunned until you cancel the ability, energy runs out or the Ghost is too far away)
— Pioneer (Sarah Kerrigan)
- new weapon: 14mm AP (normal damage, short range)
- new ability: Vehicle Repair (repairs a mechanical unit for energy)
- new ability: Vehicle Stim (vehicles that the Pioneer is riding in can use the Stimpack ability, 30 HP per use)

Open image in new tab for a better view.