A big part of that is it's notorious enemy count. Serious Sam is mostly known for its intense, wide-spread action. Upon arriving in Ancient Egypt, his journey begins. Serious Sam is the only person who can stop Mental in the past and save the future. So mankind decide to use the "Time Lock", which is an ancient time travel device left behind by that oh so helpful ancient civilization, that can send back one person. In the year 2104, monsters attacked humanity and just couldn't be defeated. This has allowed us to travel across the universe and expand. As it turns out, humans in the 21st century have uncovered ancient technology left behind by an older civilization than ours.
Unfortunately for us the future is full of chaos and war thanks to Mental. The story starts with Sam "Serious" Stone, a super warrior from the future who fought bravely against any force. Another staple of the franchise is the way it lures the player into picking up a lone health or ammo box that spawns enemies that you have to fight as a punishment for picking up the bonus. The character of Sam is often seen as a parody of Duke Nukem.
Humor is a large part of the game and it's exaggerated nature often parodies many staples of video games such as floating weapons and spawning enemies.
Although simple at face value, the game has a lot of substance and does the few things it does well, which has allowed it to garner numerous critic's awards. Games analogous to it's style of gameplay are the Doom and Quake franchises.
Serious Sam is one of the few games to successfully feature arcade-centric first person action in a 3D environment after the arrival of sophisticated, more slowly paced first person shooters like Deus Ex, System Shock 2, and Half-Life. It is the first half in a two "encounter" series where the second one is Serious Sam: The Second Encounter. The game stars Sam "Serious" Stone, a super strong, super bad ass warrior from the future. Serious Sam: The First Encounter is a first-person shooter developed by Croteam, a small developer based in Croatia.