When you think about it, Contra's developers ripped off Schwarzenegger movies and used the monster designs of HR Giger. I mean that series isn't the most original either. Hell, Castlevania did it too. They were influenced by the works of Bram Stoker, Mary Shelley, and a many others. Both of these series owe their existence to all the aforementioned people.
While I know people are going to make the comparison at first glance, I personally wouldn't call it a complete Contra rip. Aside from the 80's style action hero running around with a flamethrower, it's not really like Contra. Contra has a more cartoony visual style, too (while I think my style is a lot more gothic and influenced by Castlevania). You also get a life meter in Parasitus.
Parasitus is partly nonlinear. You'll start out in linear areas to reach large nonlinear objective points. It's being built to be mission-objective-based. Meaning once you're at your objective point, you will have to carry out a certain number of things in order to complete the mission. For example, mission 1 might require you to infiltrate a factory via the city outskirts, and blow it up by placing explosive charges in different places throughout the basement. You will also have orders to eliminate key enemy targets (in this case, two good sized bosses). Once that's done, you will try to escape through a winding sewer system while being pursued by enemies, and being attacked by the indigenous mutant wildlife (before the timer runs down and everything blows up).
The levels are decently sized, so it will require some exploration. Once you reach the objective area, it may not always be clear where to go from there.
Edit: Here's another vid of the tesla gun -
http://thaddeus.arc-nova.org/files/teslagun.wmv(sound effects haven't been added yet)