One scale for everyone.
I like that about GURPS. (It's not the only thing I like.)
By "one scale for everyone" I mean the things that rate a character in the system are used for any character in the system. It just makes everything easier. Not necessarily simple - a system using advantages, disadvantages, skills, limitations, enhancements, and so on doesn't lend it self to completely simple.
But it does make it consistent, and gives you only one scale to worry about. Monsters have the same set of stats as humans. If a ST 20 human and a ST 20 ogre swing an axe, it's the same damage all other things being equal.
I grew up with AD&D, basically. Monsters have HD, and use a Hit Dice based system for "to hit." Humans have level, and use a level based system. Monsters have set damage, or "by weapon +" but don't have a set strength score so they'd be playing on a slightly different scale than PCs. So many ad hoc additions meant that you couldn't really apply rules across the board so easily. The ability to put things on a single scale really clicked with me when I picked up GURPS. I can learn where things sit on that scale, and go from there. I only need to learn the one language of stats and skills and rolls and apply it broadly.
Not that everything in the entire game works this way (you could make a robot a character, or a vehicle, say, and not handle them the same way). But that core of monsters and characters using the same stats? That makes my life easier and makes it easier for me to have consistent results . . . and thus less "whataminute" moments that disrupt the flow of game.