A cursory look at other people's Dungeon Fantasy games makes me think I'm actually in the minority on a lot of things. Especially in the things I don't use in my games.
What am I not using that all the cool DFers are using?
Armor from Low-Tech - We don't use the armor from Low-Tech, despite my long history of using Dan Howard's armor writeups in my own (prior) games and being a co-author on the book.
We came very close on this one. I had a handy list of LT-derived armor, and we used it in our playtest of Mirror of the Fire Demon. But in the end, the ability to use Loadouts straight out of the box, the ability to just pick-and-click on Basic Set armor in GCA, and deep familiarity with the rules and options led me to stay with Basic Set armor. I really didn't want to lose all of the ease-of-use, nor did I want to open the door to rewarding the players with the free time to compile a list of their best options by cost and weight. At least half of my players have no time to be figuring out the best options and I didn't want to do it either.
Short version: Good stuff, but required work. Didn't want to do work.
Imbuements - We don't use imbuements. Nor what comes with it, the Mystic Knight. Heck, I was a playtester on this book and I don't use it. Unbalanced? Not at all. Adds a lot of complication and makes the game more Kewl Powerz than I'd like.
Don't get me wrong - this is very, very cool stuff. I love that book. But adding it would put a learning curve (and an unknown power curve) into what was supposed to be a "let's kill monsters until we get bored doing that" dungeon bashing game. And which still is. I'd like to find a way to use this in the future in another game, though.
Divine Favor - From reading Doug Cole's "I win automatically against undead and supernatural evil" experiences with Cadmus, I'm glad I didn't go with this. I see the attraction on this, but honestly, I'm fine with the basic GURPS Magic system for healing and priestly spells. So this takes care of Antoni Ten's excellent Dungeon Saints, too. Sadly.
Techniques - Again, kicking my own best work to the curb. We're not using techniques. The reason why is a line-development issue. DF doesn't use techniques. Power-Ups does have some advantages made from techniques, but you never get partly bought-off penalties. Either you Dual-Weapon Attack at -4/-4, or you've go Two-Weapon Fighting and you attack at -0/-0. Modifiers are all-or-nothing. I do allow people to attempt techniques, but you can't buy them up partly. And I discourage people from getting too cute; better to pick one or two and buy the appropriate power-up than to have a smorgasboard of options to slow play down.
And that's besides my limitations to race and profession in my campaign.
So I'm not using a lot of what I see others using. I see a lot of Diving Favor-using Imbued Mystic Knights with historically accurate customized armor and using cool techniques. But not a lot of that in my own game, for all of its house rulings and oddness. Oddly I think my "vanilla" game makes for a good testbed for stuff I can use in future professional writing for SJG. There isn't a lot of assumption of special powers, optional rules, or equipment and moves from other books.
Some things I am using - the weapons from Low-Tech (and the upward-revised damages on a few), some optional enhancements to DF from Pyramid, many cool combat rules from GURPS Martial Arts, etc.) but not some real biggies that seem very popular. But again, GURPS is a toolbox, and I don't use every tool every time I open up the box.
How about you guys? What does it seem like everyone is using except you in your own game?
By the way, Paul Stefko was the guest for an excellent intro-to-GURPS video interview, GURPS 101. It's good stuff, although I'll nitpick and say mail hasn't given reduced DR against impaling for a long time now.