GURPS has a reputation of being overly complex. The problem is due to the core books being organized as a toolkit. This makes it difficult for novices or gamers coming over from other system to figure out how to use the game. With GURPS 2nd Edition (which I started) it was easy for ADnD players see how the system will work for them. The core set + GURPS Magic was a inexpensive investment for a near complete Fantasy RPG. However starting with 3rd Edition and continuing the 4th edition, the product line doubled down on the toolkit format. The result is a very flexible roleplayng game but one that take some work pulling the bits and pieces together for a campaign.
However underlying all this is a very solid system that makes sense for a lot of genres. Whatever issue I have about the presentation, in the art of rule design the folks at SJ Games are top notch. And GURPS 4th edition is the best designed version to date.
Anyway enough of the complaints.
GURPS is a point based system. Most GURPS 4th edition campaign start out around 150 total points. This produces a competent character comparable to a 5th level character in various editions of DnD. If you want to start a true beginning character then 75 to 100 points will do it. GURPS Dungeon Fantasy starts out at 250 pts that allows character to have more endurance when exploring dungeons. Basically the increased number of points give each type of character more ways to avoid, deal with, or mitigate danger. However combat and injury are still a deadly affair and nothing like high level DnD combat.
A GURPS characters has four attributes Strength, Dexterity, IQ, and Health. There are four sub attributes based off of the four. Perception, Fatigue, Will, and Hit Points. It is rare to vary these from their base attribute. There are advantages, and disadvantage that allow you vary aspect of these attribute, for example lifting Strength.
You also have Advantage, Disadvantages, Quirks and Perks.
Advantages are things like Combat Reflexes, Magery (needed to cast spells), Social Level, etc. Disadvantage are mental or physical limtations of your character. They included positive disadvantage like Honesty, Code of Honor, as well as the more obvious hindering ones like one handed etc. A typical GURPS fantasy campaign will allow you take up 40 points worth of disadvantages. Disadvantages give you bonus points to spend.
Quirks are 1 point disadvantage. You use them to define err... quirks about your character. Like "Always sit with his back to the wall.", "Doesn't like the color red." You typically five of these. Most GURPS GM I know only ask to define 3 and letting the other 2 be defined during the first few sessions.
Perks are 1 points advantages. Little benefits you can buy. Like Alcohol Tolerance, Deep Sleeper, Honest Face, etc.
Typically you wind up spending around 195 points as a starting character. Dungeon Fantasy is more straight forward with its use of template. This is also make easier for you those who are starting with GURPS so see what goes with what for different types of characters.
The reason I and my group like the system is that we have control over the characters we create. My point of view is that the uncertainty of the campaign is good enough for me and I like having control over where I start out.
GURPS 4th edition deals with the diversity of choices by offering templates to use for character creation. These are packages of Attributes, Advantages, etc that allow you to make a character of that type. Some are professions, other represent classic roles like Fighter and Thief. GURPS Dungeon Fantasy has a bunch of these for DnD style adventures. GURPS Fantasy has more still oriented towards more toward a low Fantasy Realism.
GURPS resolves actions by rolling under or equal to a skill level using 3d6. Skill is computer as a base attribute plus your skill modifier. For example Stealth can be bought so that it is at Dex+1. For a character with a 13 Dexterity this means they would have a Stealth skill of 14. Note that it is very expensive to buy up attributes after a certain point than it is to buy skills.
A roll of a 3, or 4 is generally a critical success, while a roll of 17, 18 is a critical failure. There are exceptions for bad skill or high skills.
Combat works with one second combat round. You can do one thing and one thing only. For action other than a move there is a possibility of taking a one yard hex.
You roll equal to or under your skill to hit. If you hit, the target will likely get a defense roll. If the defense roll succeeds then the attack was a miss. Otherwise roll damage and subtract the damage resistance of the target. The result is applied to your hit point which is generally equal to your strength. If you go below zero you start rolling to stay conscious every round. If you go below negative your original HP then you start making death checks. You generally have between 10 and 15 hit point during a campaign. Slightly higher for Dungeon Fantasy.
Extraordinary abilities are either a self-contained subsystem (like magic, psionics), or they are completely explained in the description (like super powers).
In general you can expect for a given genre (realistic or not) a one for one correspondence between what you want to try and the rule you use to resolve it. However there are way to abstract GURPS especially combat. There is an option to resolve combat as a series of contest of skills where the two side roll and the higher roll wins the exchange. Or in some cases you could rule that it wins the fight if you really don't want to fuss around with the details. The skill system likewise has option for a higher level of abstraction.
What Dungeon Fantasy is going to accomplish is take all the options and list I mentioned above and implement it in a way that is ready to run out of the box. For example the skill Electronics is found in the core book, but is not going to be described in Dungeon Fantasy as it not part of that genre. The math behind what used in DF will be available so it 100% compatible with the core books.
To me the Old School Renaissance is not about playing a particular set of rules in a particular way, the dungeon crawl. It is about going back to the roots of our hobby and seeing what we could do differently. What avenues were not explored because of the commercial and personal interests of the game designers of the time.
What are RPGs?
A game where the players play individual characters interacting with a setting with their actions adjudicated by a human referee.
Rules are an aide to help the referee adjudicate actions and to help the players interact with the setting.
Dice are used to inject uncertainty which make a tabletop RPG campaign more interesting than "Let's Pretend".
The only thing a player needs to do to roleplay a character is to act if he or she was really there in the setting in that situation.