Well, to start, I'll share my hypothesis on the nature of the Quickening.
As you all may be aware, the Earth's magnetic "North" is never constant. It constantly shifts, at about ten kilometers per year (though recently* it has sped up to forty kilometers per year) in a Westerly direction. It is projected that in a few decades time the magnetic "North" will leave North America altogether and cross into Siberia. This phenomenon is called "magnetic declination", and actually applies to more than just the magnetic North, but rather to the magnetic equivalent (declination) of anywhere on the planet.
This is a global model of magnetic declination from 1590-1990 (400 years). What this essentially represents is the planet's magnetic field. As you can see, it measures in degrees East of magnetic north. Why do you suppose it changes? Well, quite simply, nobody can tell the cause of any specific change in declination, as there are far too many variables. What can be said for certainty is that the Sun exerts a very major influence. However, the cause is entirely beside the point.
The Quickening, to my understanding, is an abnormality in a person's own magnetic field. Yes, we all have a personal magnetic field, it's generated by a complex intermingling of the processes behind our thermal homeostasis (warm-bloodedness) and our location on the Earth at any given time. Not to mention a myriad of other influences. Some individuals are simply born with an abnormal personal magnetic field (PMF), which scientifically could be the Quickening.
This phenomenon itself is not enough to trigger immortality, however. The Quickening is latent and will not activate while the body maintains a power output below a certain level (probably higher than 3,000 BTUs). (For reference purposes, when we sleep we generate approximately 250 BTUs/her, which has a heat equivalent of about 75 watts. This amount increases to 400 BTUs/hour, or 400 watts, when we are awake but sedentary. When we are exerting ourselves, our bodies can generate up to 2400 BTUs/hour (700 watts).) When these conditions are met, the prospective immortal's Quickening is automatically activated by the body's natural reflexes, conferring many qualities upon the individual. The only way this condition can be met, according to the Highlander series, was for the individual to suffer a violent death. It is worth noting that the series held that all immortals are foundlings, probably due to their abnormal magnetic field.
As the Quickening is, essentially, an enormous current of positive-charged energy that surrounds and suffuses the immortal, their body instinctively draws on it when it needs to, and slowly replenishes what it draws afterwards from the body's natural energy output -- leading to immortals feeling weaker after, say, being "resurrected" (see below). Other uses for the Quickening are to speed up the healing process, and halt aging. The first is achieved by an increase in platelets, which scab over the wound, and an increase in cell division. Normally this would shorten a person's lifespan due to the telomeres at the end of the chromosomes becoming frayed more quickly, but the Quickening also rebuilds the telomeres, which also achieves the latter of the secondary benefits of the Quickening. Normally the fraying of telomeres is one of the body's means of protecting itself from cancerous cells going rampant, but the Quickening offsets this effect the same way it affects other foreign bodies in the body (see below).
When an immortal is shot or stabbed, and the bullet or blade left behind, the body's defenses naturally attack the foreign body. The inevitable result is the dissolution of the foreign body. Normally this process would take years, even decades, but with the Quickening it can be reduced drastically. With bullets it can take minutes, with blades it can take hours. Even if an immortal is "killed" by a bullet, the Quickening will still dissolve the bullet and then revive the immortal in the same way a defibrillator does: delivering an electric shock to the heart. That is why immortals who have died "wake up" with a jolt.
This same effect applies to viruses (including diseases) and poisons. This is why immortals don't get cancer, as the cancerous cells are quickly destroyed by the body's energized defenses. Sadly, the Quickening also is lethal to the fertile head of sperm, regardless of the source.
The Quickening, as stated, isn't merely contained in the immortal, but also surrounds them. The range the Quickening extends around the immortal varies greatly, though the result is always the same when one immortal's Quickening intermingles with another's: a brief change in the polarity of their respective Quickenings as they become equally charged. This translates into a powerful sensation that alerts the immortal of another's presence. When an immortal is killed, his Quickening is no longer anchored to him and escapes into the nearest most powerful source that attracts it; namely, the other immortal. Only a bare fraction of the deceased immortal's Quickening actually makes it into the victor, and the rest is discharged in a terrific lightning storm. The entire event is also known as "a Quickening".
So why don't immortals fight on holy ground? During the medieval times people used "divining rods" in order to locate holy ground to build churches. These rods are attracted to magnetism, and therefore "holy ground" is a site of an anomaly in the Earth's magnetic field where magnetism is enhanced. Thus, a Quickening that takes place on holy ground is liable to be exceedingly powerful, and likely to incinerate the immortal or otherwise bring about disastrous consequences (such as the eruption of Mount Vesuvius and subsequent destruction of Pompeii in 94 A.D., as mentioned in the Highlander series). It is worth mentioning, however, that in Highlander: Endgame (which notoriously violated many previously stated absolutes from the series) the original version had Jacob Kell kill a room-full of immortals on holy ground with no perceivable consequences.
The Holy Ground rule is more easily given a scientific explanation than a rule I had neglected to mention. That is, that one immortal may challenge another, and no others may interfere once the fight has been engaged. Many times throughout the series (and even in Highlander: Endgame, which also violated a number of previously stated absolutes) this rule is violated, with no perceivable penalties or demerits assigned to the immortal who "cheated". We can pretty much assume that this rule has the same merits as the last of the three rules to the Game: in the end, there can be only one. When is the end? Nobody knows. The Gathering is a time when the last immortals will come together to fight for the Prize, but it is supposedly a long ways off, as there are thousands of immortals left in the world.
So why don't immortals simply rule over the world? Well, the series holds that some of them did. The Watcher Organization was founded by Ammaletu the Akkadian after he saw Gilgamesh the King coming back to life c. 2,500 B.C., and Darius the Priest (50 A.D.-1993 A.D.) was once a great general who could have ruled Europe for a thousand years had he not taken the head of the oldest immortal of the time (1,500 years ago) and become good. Immortals in the modern day live secret lives, seeking to avoid bringing back the witch-hunts that were rampant in the middle ages.
* "Recently" in astronomical terms. It has sped up gradually over the past century.
--------------- Added 15 Jul 2008 at 08:42 pm ---------------
Part Two: my suggestion for the thread's setting...
As I said, immortals today live secret lives in fear of a resurgence of witch-hunts. Few take up positions that would draw them the public's attention, though many ill-aligned immortals have become lords of crime cartels that run underground movements. I did a little thinking on how I would write a story for Highlander, and I made up Watcher accounts of global statistics. The way I had it work out was that there were approximately 150,000 immortals on Earth. More immortals are born every day around the world, though that approximate amount is maintained because roughly the same amount of immortals kill each other every day. A thousand years ago there were more like 40,000 immortals in the world, and two thousand years ago there were more like 20,000. Methos, the oldest living immortal (who most immortals consider to be a myth, but still a fair few try to find for a chance to take his head), who is over 5,000 years old, was among possibly as few as 1,000 immortals during his early life.
I had also made some statistics for the age brackets of most immortals. Young immortals (under 100) are often killed before they get a chance to make their way in their new lives. They account for approximately 20 per cent of the total immortal population. Those who are talented enough and are fortunate enough to find a teacher interested in teaching them to defend themselves before running into someone who just wants their head may grow into mature immortals (200 to 400), who develop better combat techniques as well as insight into life. Approximately 35 per cent of immortals in the world fall into this category. Experienced immortals (400-1000) are highly capable and confident, and account for 30 per cent of the immortal demographic. The last 15 per cent are ancient immortals (1000+). These individuals are often of incredible caliber (ex. Kalas) or guile (ex. Amanda); or both (ex. Methos, Kronos, Rebecca). Some have unique gifts that give them an incredible advantage (ex. Cassandra).
Of the whole 6 seasons of Highlander, there are only a few ancient immortals who make an appearance, most of whom had more than one episode devoted to them... and most of whom are dead (many of them killed by Duncan, which says volumes about his fighting skills).
-Darius (deceased; killed by James Horton)
-Grayson (deceased; killed by Duncan MacLeod)
-Ceirdwyn
-Rebecca (deceased; killed by Luther)
-Kalas (deceased; killed by Duncan MacLeod)
-Methos
-Kronos (deceased; killed by Duncan MacLeod)
-Caspian (deceased; killed by Duncan MacLeod)
-Silas (deceased; killed by Methos)
-Amanda
-Marcus Constantine
-Nefertiri (deceased; killed by Duncan MacLeod)
-Haresh Clay (deceased; killed by Duncan MacLeod)
-Kyra
-May-ling Shen (deceased; killed by Michael Christianson)
Most of the other villains fall under the 'experienced' category, including the notable Xavier St. Cloud (into his 8th century), who was one of the most frequently occurring antagonists until Duncan finally cut his head off, and Kenney, an eight-hundred year old child-immortal who nearly killed Duncan using the same scam he had used for centuries (playing innocent and lost so other immortals will take him in to protect him... and then taking their heads when they turn their back for an instant).
There... some additional context for building your characters.