For all the hoopla about how GURPS is an insanely complicated game, it pretty much all boils down to a single rule: roll a target number, usually based on one of a character’s capabilities modified for the circumstances, or below on 3d6. That’s it. I’m not stats geek enough to care specifically about roll low vs. roll high or 3d6 vs. 1d20 vs. 1d100; the point is that it’s a single, standardized die roll. Such complexity as GURPS has in play is about figuring out which capability to roll against and how to modify it for the situation, but those are a necessary consequence of one of GURPS’s other virtues.
The point-based system provides a single currency and relatively open set of choices for developing a character, as opposed to class-and-level systems or lifepaths, which, while they’re not without their own charm, tend to be more rigid and complicated in practice.
Attributes (and Talents) and Skills
In GURPS, you have attributes, which indicate broad areas of and ability: how agile you are, how much general mental capacity you have, and so on. Then there are talents, which are somewhat more focused indications of being good at things: how good you are at the arts, how smooth a social operator you are. Then there are skills, indicating your ability in limited, specifically trainable areas: how well you drive a car, how good you are at chess. There’s a strong relationship between these. Attributes and talents form the basis for skills, moving pleasingly from the general to the specific. In other games I was playing at the time I discovered GURPS, this was a welcome change. It made attributes matter in play in a big way, as opposed to other games I was playing at the time, where attributes might provide modest bonuses at the extreme end of things, but rarely came up.
It’s a small thing, but again a big change from other games I was playing at the time. I was playing a lot of games where muscle-powered weapon damage was inherent in the weapon: Sword X does, say, 1d8 damage, with maybe a modifier for strength. That meant a giant using a dagger did next to no damage, probably far less than if he used his bare hands. GURPS turns this around, damage is based on the strength of the user, with some modifications for the type of weapon. It’s a small thing, limited to a fraction of situations in the game, but I found it revelatory.
As Big As You Want It To Be, But No Bigger
Here’s the thing I really like about the GURPS line, as opposed to the above points which are about the GURPS rules: it tries to cover everything. If I want rules for armies fighting armies, I’ve got them. If I want rules for building a starship or defining a solar system, I’ve got those, too. Building castles? The finer points of wrestling and use of firearms? All there, along with premade campaign frameworks saving me the trouble of helping players build characters for certain genres, elaborate rules for social interactions, a variety of systems of supernatural abilities and even for building systems of my own, and so on. If I feel I need it, it's right there at my fingertips.
On the other hand, it's all optional. I don't have to use any of it. And for the most part, I don't. I usually run fantasy, so anything which touches on technology after the Renaissance is rarely useful for me. The second most common thing I run is cliffhanging pulp adventure, so modern and SF-related works still aren't that useful to me. And I don't feel a need to add to the combat rules in the Basic Set or move away from spell-based magic, so there are other large sets of rules I never touch. I have never once used GURPS Powers in anger. And my game doesn't suffer for it. But if I want to move in that direction (say, move back into space opera), it's there and ready for me.