We're in the third session now, and this is clearly the most extensive game of GURPS I have run in ages, certainly the first extensive 4e game. Some of the character of GURPS has begun to stand out to me.
GURPS Is Very Powerful: The first thing that stands out to me is the sheer power of the engine. When Walter complained about the lack of Vehicle support, I delved into the book and found the vehicle support. The same was true of our pop-up question. If we want to know something, the rules are there, just waiting to be found. GURPS effectively covers every situation I know of. It does miss out on some more obscure situations (designing guns for Quetzali is a little difficult), but it provides more support in those situations than most games do.
GURPS Is Reality Based: This is an obvious one to anyone who has played for awhile, but many games handwave rules and slip into a narrative mode. If a WotG character wants to carve his initials on someone, well sure, why not? In GURPS, though, the GM will ask "Did you take a skill for that?" Likewise, you cannot expect your character to just wade into battle and walk out alive. You have to think carefully. Most of the complaints I've seen in the game so far have erupted from arguments over how something would really work vs how it should work in a narrative game (I've been on both sides: Walter's complaint about the lack of vehicle support was reality based, Byler's complaint about being unable to just fling around a guy with a Judo roll before he got shot in the temple was narrative based). You don't have to stick to this model, but it's certainly GURPS's default mode, and if you don't explicity change things, this theme tends to leak back in.
GURPS Rewards Deep Investment: One thing I've noticed: most of my players don't know much about their technology. Roomie has probably researched the most about his character and actually knows at least one ammunition type by name (and uses nothing else), and knows what his drugs do. Nobody has really investigated or invested in the possibilities of the Radar system ("You mean the imaging radar can resolve people at a one-mile range?"), or the Computer system ("Can I get software to boost my Electronic Ops (Sensors) skill?"). You need to know what your character can do, all of his little powers and perks, like the back of your hand. This, incidentally, is what gives rise to accusations of GURPS twinkery: the average player just plays, while the twink investigates everything in the books, and thus comes up with ideas that nobody else would have come up with. It's also why having one well-behaved twink is a good thing, because they teach the rest of the players how to use the system. I'll have to remember that.
Corrallary of the Above: GURPS really is Complicated: Given that you need to spend lots of time figuring out how everything works, the reality of the situation keeps slipping in, and GURPS has so many little rules at your disposal that it can easily overwhelm someone. Compared to, say, the WoD, I'm still learning new things about GURPS, still working hard to gain full mastery of the system. WotG, I think, is comparable in complexity.
Fortunately, GURPS is Resilient: Obviously, I don't know the system, but even not knowing it, even making mistakes every session, the game is still very fun. Most of the games that I dislike, that I point to as problematic, easily break down if everything doesn't go exactly to plan. GURPS is designed without an explicit plan, so tends to cover a variety of situations well. You can't just walk into it and have it work like, say, WotG, but a relatively small amount of knowledge and a little GM fore-planning will ensure you can miss most of the book and still run a good game.
GURPS takes alot of Paperwork: Fatigue, Hitpoints, character points, lots of skills, facing, retreating by one step, when you can and cannot use Luck, and so on. It's no worse than WotG (which does most of its record keeping with Dice and Beads, but still), but I'm glad we're playing GURPS on the computer.
GURPS Rewards Patience: This is true both from a meta-gaming and in-game stand-point. One thing I note is how seldom players bother to aim, lock on and build up their modifiers. If they were, they would gain a huge benefit (the NPCs do this, and that's when they get scary). Likewise, I put Shawn on hold for his character so I would have time to design his loadouts and his template. Once those are designed, they are useful for anyone and everyone. Taking a little extra time before a game to get things set up really rewards the rest of the game with smooth, thoughtful play.
I Like GURPS: I've liked it for a long time, but much of that was based on the quality of supplements, rather than actual gameplay. Someone once asked us to compare RPG systems to women, and I likened both WotG and GURPS to girls I really liked alot but never had the time to date. I'm kinda glad I've taken the time to really get to know both, that I'm playing both, and that neither has failed me.