[Anaraxes]

3d6, roll low for success. Much simpler than D&D. Lots of modifiers for different circumstances? Sure -- but D&D has its own long lists, plus generally classification into different buckets of modifier types (circumstance, status, armor, item, etc) and stacking rules for which ones combine.

Able to build the character you imagine, rather than play someone else's out of a splatbook. (See the Beauty of GURPS thread. Does involve knowing character creation, but picking Advantages isn't really more complicated than picking synergistic sets of feats.) Character budgets afford some room for color without gimping your main abilities.

Less dependency on the "stack the one trick pony until the system cries uncle" design. Blended character types are still viable, if not the best at everything they do. In most versions of D&D, mixing in anything outside the designated path means you gimp your character. (The exceptions are usually the broken paths through the D&D prestige class splatbooks, where if you happen to do exactly the right thing every level for eight or ten levels, you can activate all the right special features for synergy godhood. And you thought picking Advantages was hard? :))

Tactics pay off. Very often in D&D, the most effective thing to do is just attack repeatedly. Kill them before they kill you. Many more tactical options in GURPS, and choices do make a difference. (Some players will find that a disadvantage, because making choices means you have to know something about those choices -- not to mention actually choose.)

Bribes for RP choices (Disads and Quirks).