GURPS

Character creation in GURPS is detailed, but slow.  There are no levels or classes, just points, which can be used to buy attributes, advantages, disadvantages, or skills.  Different campaigns start with different amounts of points, so that (for instance) a superhero can start out as a more powerful character.

Skills are how characters accomplish things in GURPS; for instance to shoot someone you would want the "Guns" skill.  Skills are based on two of your attributes: if you improve your Dexterity, for instance, you similar improve your chance to ride a horse, shoot a gun, or do anything else physical.  The remaining two attributes set how strong and tough the character is.

Advantages cover any non-skill benefit a character might have, such as a high pain threshold, wizard training, or the ability to move objects with their mind.  Advantages can also be further be customized with enhancements or limitations, allowing you to gain more or less powerful versions, that cost more or less points as a result.  This can allow for a nearly any power you can imagine to be described as a GURPS advantage.

Similarly players can also take disadvantages for their character, such as Code of Honor or Blind, to gain extra points.  All together this system allows for incredibly individualized character, including almost any you'd want to re-create from fiction or real-life (in fact, there are multiple GURPS books with stats for historical NPCs such as Ghengis Khan).

The main problem with GURPS's creation system is simply how detailed it is: it takes awhile even for experienced players to make a character, and first time players may find all of the options a bit overwhelming.

To succeed at an action in GURPS, players roll 3d6 and attempt to beat a target number, which varies depending on the action (typically equal to or under the character's relevant skill).  This results in a far more predictable distribution of rolls: just as when you roll stats for a D&D character and (usually) get mostly average results, rolls in GURPS will have a far more average spread of results than (say) a D20 roll in Dungeons and Dragons.

However, there are a lot of details in the GURPS rules.  Although it's not required, if you want you can play GURPS with miniatures and a hexagonal grid.  Again, a hex grid may seem strange to players of D&D and other square grid systems, but the choice eliminates an issue that square grids have, with diagonal movement being faster than horizontal/vertical.

GURPS also has a great number of situational rules.  For instance, to make a ranged attack you can't just roll under your shooting skill: you also have to consider your target's distance from you, and their speed, then consult a table to get the appropriate modifier.  Just like the rest of GURPS, this results in a more-realistic feeling system ... but it also makes GURPS a more "crunchy" system, which will take a bit longer to learn than other options.

Hexagonal Grid

GURPS uses a hexagonal grid for combat.  Hexes offers a benefit vs. square grids, when it comes to diagonal movemen: in square grid games (eg. D&D) characters moving diagonally move more quickly ... but since hexes don't have corners, they don't have this problem.

Initiative

Unlike most games, GURPS does not have a random element to initiative.  Instead, characters always move in order based on their basic speed (a stat derived from their Dexterity and Health scores).

If one group surprises the other they can potentially get several seconds of actions before their opponent can react ... and since a single round of combat in GURPS lasts a second, that can be significant.

Maneuvers

On their (one second) turn a a character can take a single maneuver, plus any number of free actions (eg. talk or drop an item).  A maneuver could be to move, attack, or move and attack (at a penalty), as well as other options such as aiming or feinting.  A character who moves can move a number of yards equal to their basic move (ie. basic speed with fractions rounded off).

Attacks

To attack a character makes a skill check with the appropriate weapon skill, applying a variety of modifiers for things like the target's size, range, and speed, or the weapon's accuracy bonus (if they took the time to aim), the lighting conditions, any cover the target has, etc.

Active Defenses

If the check succeeds the target can then opt to make an "active defense". There are three options: Dodge, Block (eg. with a shield) and Parry (with a weapon), and ranged attacks can only be dodged.  Active defenses are also a skill check, for Block and Parry; to Dodge you simply roll your Basic Move + 3.

If the active defense roll succeeds, the attack misses.

Injury

If an attack roll succeeds the attacker gets to make a damage roll, which will vary based on the weapon they use and other factors (eg. ranged weapons do half their damage when far enough away from a target).  If the defender is wearing any armor they can subtract it's "DR" (damage resistance) from the damage, and the rest goes through.

Depending on the type of damage dealt, it may be multiplied (eg. cutting weapons deal 1.5x ), and then the defender loses that many hit points.  Hit point loss can then trigger other effects, such as shock, stun, and major wounds ... and at 0 or below HP you have to make Health checks every round just to stay concious.