One thing I feel is worth mentioning is that tactics are far more important in GURPS. A bunch of mooks using good tactics can possibly be as tough as a combat powerhouse who uses poor tactics.
Beyond that, skill levels are going to matter. Enemies with poor skill are not going to be as tough as enemies with expert levels of skill. The chapter in the Basic Set which talks about skills should talk about that. I believe GURPS Light touches upon it as well, but it's been a while since I've looked at Light, so my memory is a bit fuzzy.
You're correct in thinking modifiers to skill will be used instead of DCs. Modifiers usually range from -10 (extremely difficult/virtually impossible under most circumstances) to +10 (so easy that you may not even require a roll.) There are also modifiers to skill based on conditions such as darkness; aiming for a specific body part, and things of that nature. Modifiers typically do stack up.
So, let's say I have a skill of 15 in broadsword. I declare that I am swinging for my opponent's leg which gives me a -2. I would roll against skill 13. Now let's say I try that in a dimly lit room. (We'll say for sake of arbitray example that the GM decides there is a -3 penalty for darkness.) I would roll against skill 10.
I'll also add that GURPS combat is less forgiving than D&D 4E combat (D&D is pretty easy on the PCs when it comes to a lot of things.) That's not to say GURPS is harsh, but compared to D&D, it may be perceived as such. Though it's important to remember that GURPS does not use static defenses; you're not expected to just stand there with a static AC number and get hit. The defender usually has the option to dodge, parry, and/or block.
One of the upsides I personally found to have come from the way GURPS treats combat is that other options become more attractive than they are in D&D 4E.
I'm sure others will chime in with more advice. I'm away from my books and notebooks at the moment. I'll try to add more later. Though, personally, I tend to just think out in my head how I feel an encounter would actually play out 'for real.' Yes, I know things such as fireballs and dragons are not 'real,' but I find it helpful to play out the scenario in my head as though the elements (even the fantastic ones) in a combat were being used in an actual real life encounter. Things such as massive numbers matter much in the way you'd expect them to in an actual fight.
Oh... don't bother to stat out every detail for mooks or faceless NPCs if you don't need to. It's often plenty enough to know that Guard #3 has X skill in a weapon, Y hitpoints, and defenses of Z.
Be sure to check out the Steve Jackson Games fora. The people there were extremely helpful when I started GURPS.
