Enjoyable kick and sound for the guns. Funny ending, and great voice acting. The final level was disappointing, since the railgun splattered the spawning portal into bits with just a few shots though.
Repetitive gameplay and low difficulty. The slow mobs collect, which you can just run in circles to pick off. The rail gun steamrolls everything with its fast reloading and instant kills. Enemies do pretty much the same damage over time when they touch you, rather than static damage, with animations. Most enemies differ slightly in size, speed, and appearance.
If the enemy types differed more to be unique challenges, like hopping, burrowing, ranged attacking, splitting into smaller units since there are already smaller versions, or even group tactics like encircling the player, it'd be much more enjoyable. Also, if levels changed the environment as the game progressed, that'd be great too.
For the player, maybe allow holding two weapons, and the ability to drop them? If at the start of each new levels, the woman had a little witty banter with the marine, it'd add the story element that's missing. Say like how she increasingly gets worried, so it builds up to her realization at the end.