sword-dancer,
Not to be obtuse, but doesn't your point help prove my point?
GURPS, in that respect, is like D&D.
You can do GURPS basic, and have things go reasonably quickly. Or you can add in the additional rules, and have things get complicated.
D&D doesn't have swashbuckling rules or martial arts rules -- there are third-party options there, but those are, effectively, published house rules. GURPS Swashbuckling or GURPS Martial Arts are official GURPS material, at least as official as the splatbooks are to D&D, and far more so than any third-party supplement.
So if you say, "I want to run a campaign about a bunch of European-style ocean traders and merchants and pirates who have just started to come into contact with an Asian-style empire to the East," you need GURPS basic, GURPS Swashbuckling, and GURPS martial arts. Or at least, if you want that fully customized and configured game, you do. Yes? No? Those additional books add new rules, change the combat engine (?), and do other things that raise the learning curve for someone who wants to join your game. With D&D, the core engine is always going to be the same, and it's just an issue of the DM telling you that this feat or that feat is or isn't allowed in the world, or that all monks have to come from the eastern lands, or such. There aren't any changes to the core engine, the core mechanics. You don't have to learn a new style of combat to play a monk.
What you DO have to do in D&D is figure out a lot of Feats -- but these feats were, many of them, taken from GURPS to begin with. So comparing them unfavorably to GURPS as too complex doesn't make sense to me.
(Again, I welcome disagreement. I don't know GURPS well enough to say all that with any authority, and I should be smacked upside the head wherever appropriate.)
What tends to make combat more complicated in D&D is feats -- which are easily added or substracted, and which rarely if ever change the core functioning of the rules engine. Closest I can think of is Weapon Finesse, which changes melee combat from BAB+Strength+(bonuses) to BAB+Dex+(bonuses). Nothing changes the way you calculate your armor class, the way you overcome spell resistance, or the way you roll a saving throw.
So saying that GURPS basic is easier than D&D is like saying that D&D without feats or skills is easier than GURPS basic -- it's true, but it's a comparison between a stripped-down version of the rules and a full-featured system. Which hurts flexibility.
-Tacky