It doesn't unlock any extra equipment or areas. The locked doors you find can open when you find the tool for them (eg. plasma torch, ion touch, gas mask, level 3 security access tuner). These tools are required to progress through the game, but once you've found them, you can also revisit locations to open doors and find collectibles and pickups that you couldn't reach the first time around.
The differences between easy and hard are:
- Enemies give and take more damage.
- Enemies have better senses, so you'll be spotted more easily.
- The alien sticks a lot closer to you, so you have to make distractions to move forward when it's hunting you.
- The alien is more aggressive, and quicker to adopt tricky behaviour such as entering human-accessible vents and checking under tables/lockers.
Nightmare difficulty adds a few extra caveats - motion tracker is nearly unreadable due to interference, no maps, no HUD, much fewer salvage items and ammunition, and even more aggressive alien behaviour. Basically it's for players who already know the game inside out.