You don't carry any weapons. This forces you to do something more than just hold down the fire key. Don't worry though, you aren't completely helpless. Then you can cause them to explode, or play as the Slig for a short time thereafter. This is a significant advantage when used properly.
Enemies seem to think for themselves and some even have personalities.
The gamespeak feature helps to add even more realism. It allows you to actually interact verbally with the other creatures, with a limited vocabulary. This is a feature not found in other games and allows for a more realistic problem solving approach. Alright, alright, it has some down sides. There is hardly any character development during the game.