I've been thinking about why I enjoy playing and running GURPS so much. I started out playing D&D and it's still a native language to me, but I don't run it or play it. I switched the Rolemaster and I enjoyed that, but I don't play it anyway, either. Why not? Well, here's why I enjoy GURPS so much.
Combat is potentially lethal. One good hit, or one lucky hit, can kill most PCs in most situations. While high character skill and clever PC tactics can ameliorate the risk to some extent, it doesn't eliminate this. Supernatural traits can eliminate risks to an extent, but only a very few expensive ones can remove the downsides to combat.
Low-level D&D was lethal, too, but your HP score would rise and make it less and less lethal. You need save-or-die to make up for this, and increased monster damage. GURPS pretty much stays lethal the whole time. This really pushes the importance of clever tactics and player skill (see below) to minimize the times you'd playing chicken with the law of averages.
You can design what you want. The key aspects to point-buy systems in general are that you have equality of choice, and can choose.
While rolling up a random guy is fun for me, the fun wears off a lot. Plus as amusing as 3d6-in-order can be I like to decide what kind of guy I want to run and give him a whirl. It's jarring to me to have a character concept in my head and then roll up something different. Traveller might kill you in chargen, even, and your concept of "Luke Skywalker" might turn into "40 year old ex-Marine" in no time.
You can start at any power level your want. While D&D has been described as being a system where you can be Elric or Conan or Gandalf or Fafhrd or the Grey Mouser, you can't. You can aspire to be them deep into the campaign, and then only if you roll up the right stats and roll well on your saving throws. GURPS lets you choose where to start. My current Dungeon Fantasy game is 250 points plus 50 in disadvantages plus 5 points in quirks. My old pirate game was scads of 50 point guys plus a few choice 75 and 100 pointers. I've played power levels in between often. You really can stat up Conan if you like, and just run adventures with him, no waiting or build up.
Player skill matters. Don't let the skill system and Perception checks fool you, player skill matters greatly in a lethal system. I like this a lot. Combat is lethal, damage can last a while and impair you greatly. Consequences for failure are realistic, which means falling 10' can be really bad, burns suck, and mishandling a grenade unpleasant at best. But a good player can use this to his or her advantage. GURPS is a game system where slicing the pie is a valid and useful tactic, throwing flaming oil can be a fight ender if you do it right, and using your head is rewarded. That lethality goes both ways, so your brains can maket a fight totally unfair in your favor, or kill you if you get overconfident.
One system to rule them all. Or at least flexibility to cover most things I want to do in gaming. We can keep rolling 3d6 whether it's pistols or swords, fists or social repartee. We don't need a new system and we don't need more rules. This is why I've run a fair cross-section of games, including low-power high-lethality pirates, fantasy games from low to high power, and post-apocalyptic madness.
The really fun bonus part to that flexibility is that my players know that deep down, I really could have aliens attack my Dungeon Fantasy game without any extra work - I'd just grab GURPS Ultra Tech and pick out blasters for the UFOs.
It's simple. It really is. There are a lot of rules for special cases, but the AD&D Dungeon Masters Guide is rightfully infamous for being full of tiny special case rules and stuff you didn't notice you did wrong until years of read-throughs later. GURPS is a toolbox, and you can pick and choose tools when you want them and need them. You can drill down to extreme details in a critical one-on-one duel or fuzz out the details for fast resolution of a big fight. Yes, I'm unashamedly responsible for some of these rules, too, so I'm biased in their favor. But even I don't use all the rules I co-created all the time. You don't need to. People can be intimidated by the sheer amount of rules, but there aren't really a lot in play at any one time. And you can freely ignore most of them most of the time. But you've got a whole system of consistent and reasonable rulings to answer questions that you as the players of the game might not know the answers to. How long to climb a ladder, how high can I jump, does the king believe me, how much does my guy know about liches? It's easy enough to dig deep or you can just wing it.
Anyway, that's pretty much why I like GURPS so much.