Characters:
Stats and skills have values between 1 (poor) and 5 (legendary), although some beings may have higher levels
Six body stats: constitution, movement, strength, senses, dexterity and agility
Three spirit stats: intellect, charisma and will
Additionally, some derived stats like hit points, movement speed and defense modifiers.
Personality - what is most important for the character, three aspects: honour, adventure or reputation
Skills - a lot

, each bound with a particular stat.
Destiny - either randomized or chosen by the storyteller - e.g. the hero may be destined to become very rich, or to live in infamy
Races - human, elf, dwarf, gnome, halfling and witcher (yes, witchers are treated as a separate race). Races have different min and max stat levels, as well as different starting skills
The stats and skill levels are purchased with points (although there is a die-rolling method for generating stats available), separate pools for skills and stats, the skill pool is dependent on intelligence and will.
No levels - instead the character gathers experience points and may purchase new skill and stat levels with them.
Tests - roll as many dice, as the value of tested stat. One die is a fate die - 6 on it means an extraordinary success, 1 is a complete FUBAR. 4+ is a success, anything else is a failure. You need a number of successes (between 1 and 7) determined by the storyteller to do something. Skill level reduces the required number of successes. In the case of "head to head" tests (trying to be better than someone else), the required number of successes is raised by the amount of successes by which the other person has exceeded his goal.
Combat:
To hit someone, you have to make a successful test of a combat skill. The test difficulty (required number of successes) is equal to enemy's defence (his defence modifier + his weapon skill for close combat, defence modifier + his shield in the case of ranged combat) .
Combat Points (one of the derived stats) allow you to increase your odds of success - before the roll you may declare to spend a few CPs, each of them allows you to raise result on one die by one point. You can't spend two CPs on the same die. CPs may be regenerated by sleep or meditation. Witchers have another stat called Power Points, which allows you to do the same.
Combat Is divided into rounds, in each the characters act in initiative order (initiative is decided by D6+senses). Characters may declare delaying their actions instead of acting when they're supposed to.
Wounds increase the difficulty level of skill tests. The location of a wound is randomized (although you may aim at a body part - it raises the difficulty), you track both the health of the character and health of his hit locations. If a location is damaged too much, it will be useless (disabled arm) or the character may even lose it. The amount of wounds a character may survive is determined by its constitution, on which the derived stat for hit points is based. A character may have between 23 (poor constitution) and 35 (legendary constitution) hit points. It's not much - a sword will cause D6+2xStrength+other modifiers damage, two-handed weapons even more (2D6+2xStrength+modifiers), so a character will be able to survive only a few hits. Each "surplus" success on attack test adds +3 to the damage. "6" on the fate die is a critical strike - damage is doubled. Hit in the head - damage is doubled, tripled in the case of critical strike.
Armor and shields protect from some damage - between 1 (for leather armor) and 3 (plate armor, tower shield) points but may reduce some stats and skills.
Combat maneuvers - you have to buy most of them as if you were buying skills (although a few were available for free), and have to spend Combat Points to use them in a fight. They may modify the defence of both the character and its opponent (as they may require more fancy maneuvering or simply be difficult) and provide "special effects" - e.g. a Painful Strike (used in unarmed combat) will add +1 to the damage caused by the strike and also will stun the enemy for D6 rounds if he doesn't pass a Vigour (skill) test with the difficulty equal to caused damage (all that for the price of 5 CPs), while the Feint (for armed combat) maneuver will cost you 2 CPs and will simply lower the enemy's Defence by 1.
Magic - wizards, priests and witchers may use it. They have different sources of magic (witchers use their amulets, wizards - amulets and elements, priests - their gods). The various spells are used just like skills, but they of course require spending power points to use them.
As for "how Witchers compare to other characters" - since you had to spend skill points just like everyone else, they were not "better"

. They have high minimum stat levels, good set of starting skills for a combat-oriented character and the ability to use magic and drink elixirs. If you selected a small set of skills and stats and focused on them, you could easily create a killing machine. If you tried to develop everything at once... well, no witcher lives forever.