In my humble opinion, GURPS has many advantages over most other popular RPG systems. 

  • There are no classes to constrain your character concept. I'm sure that all you roleplayers have sometimes wanted to have a character based on a particular concept, or on a character from a given novel or movie, and found that you had to wrestly with the system to get what you wanted. 
  • The rules are based on reality, and undergo reality checks during playtest.
  • The rules are well playtested, edited and documented. How many role playing books do you know with 2+ pages of bibliography?
  • The rules are generic. This means that they aren't constrained by the realities of a given setting, but designed to have broad aplicability.  
  • The rule books contain rules, the non-rule books (Genre books, setting books) contain information on the genre/setting that isn't usually tied to the rules. Thus the non-rule books are usefull for any game system. Some people buy GURPS books to suplement their non-gurps campaigns! in fact I've heard an alternate interpretation of the GURPS name: Generaly Useful Role Playing Suplements.
  • Character creation is point based. That is, you have a pool of points, and use them to create your character. No need to roll dice to generate your atributes, you will play the character you want to play, and not what your luck allowed. 
  • Character creation might seem dificult and complex, but once you get used to it, it is in fact rather quick. The actual play is much simpler. The core mechanic is roll 3d6, and compare it to your skill level. You can't get much easier than that!
  • The system is modular. It offers different resolution aproaches. You can tweak the system to make it as gritty realistic or as cinematic as needed. Also, you don't have to use the rules, most of them are optional and recomended for Game Masters hat like the extra detail.
  • The system uses an Advantage/Disadvantage/Skill division. Any trait falls under one of those categories. Combat, Magical and Social skills are just that, skills!
  • You don't have to learn a different game system for each game you play. GURPS can be used in any setting. I have, for example, used it in traditional fantasy (Banestorm), comic fantasy (Diskworld), realistic modern day investigation, superheroes, and hard sci-fi (TransHuman Space)