I think this is interesting to read, even if it feels like a long walk down memory lane. I'm coming from this with an extremely different point of view than most, as I'm pretty exclusively UC and more likely to frequent PA than fanatics, but I figure I'd add a bit to the polarism history as I do know a good deal about it (I might also do a bit on Tess, but I've been putting off working out for a while already.)
Polarism, when the show was on, started small (like any UC on any show) and grew larger as the series progressed, and continues to grow years after the show ended, probably because more dreamers/candies, just like anyone else, have gotten over the initial eww of their OTP being split up. I think the first fic, and definitely the first influential fic of note, was The Polar Novel. TPN was great because it followed the show and made a secret romance between Michael and Liz believable and forced a lot of dreamers to "look closer" and examine some of the subtext that was happening between Michael and Liz. And, I may be biased, but there was a lot of subtext.
I think the first wave of polarism tended to follow this trend -- fics that started with that line in missing -- "thank you for giving me another reason to envy Max Evans" and gave their own spin/what if scenarios. This was pretty much par for the course in season 1 up to and through Destiny and important writers during this period included people like Gaudicia, DMartinez, White Otter (obviously), Bennie and the Three Amigos -- Belinda, Lee and Polarist. I'm sure there were others, but I'm bad with names both online and irl, so I apologize.
Somewhere along the line, the first PA (which was on ezboard) was created and was a small place for polarists to congregate. Most fic at this point was pretty smutty (not unlike the vast majority of rosfic in general), fluffy and pretty short.
The summer after Destiny, polarism continued to grow -- there were a few "Liz is Michael's destiny" fics (Gaudy and Belinda were two of the first with this trend), but most of them were built off of the hug in MTTM and there was a larger number of true AU fics that came out that summer -- etoile1 was one of the many au writers that started making herself known during this period.
After EOTW, polarism began to explode. Most of the people who converted to polarism during this period were former dreamers (more aptly, probably, Liz fans) who were disgusted by Max's behavior following EOTW (and later on, the Tess sex) and were basically done with him. Stories about Liz losing her virginity to Michael and then realizing that she loved him were pretty common -- and following the Rath/Liz kiss, there was a whole subset of Rath/Liz that spun out of polarism. Maria was also beginning to take over for Tess as the "default baddie" in polar-fic, but since there were so many ex-dreamers Tess was still public enemy #1 in polar fic (again, not unlike the vast majority of rosfic at this point).
A common trend in season 2 (and onward), was the simultaneous unrequited desire that both Liz and Michael had for one another. It would come up randomly -- Liz always having a thing for Michael and Michael wishing he had been the one to save Liz. But they'd never act on it until some serendipitous moment where it all came together.
Another trend that came to fruition during season 2 was Max bashing. I think some of the one of the more illustrative moments included having him dismembered rather graphically (something I take credit for), and highighting some of the "darker" sides of Max's personality -- his controlling nature growing into Max being abusive to Liz/Michael. Brendanfehrobsessed, Lucky Star82, Stormybear30, and I started writing during this period.
VLV and the Tex storyline were great pieces of fodder for polarism -- Gaudy's After Vegas is definitely one that I remember rather vividly. Obviously, Departure gave birth to the successor to White Otter's legacy in Polar fandom -- Peachykin's Memory of Her. Darkvixen who is the most prolific polar writer (with over 50 stories to her name) began to write at this point, bringing a great deal of angst and insight into Michael. JadaLyn and Lauren began writing as well.
Season 3 was more of the same -- great writing, great exploration of subtext. Trends that started during this period included: Liz doesn't get back with Max after Departure (basically telling Max to take his spawn and shove it), Liz and Michael after BTM, and Liz and Michael go further during TOTP. I'm pretty sure that Robyn (2x4) and Queenie started writing during this period.
After the show ended, unlike mainstream Rosfandom, there was a great deal of post-Grad fics. Most of them featured dark!Max growing more sinister as things spin further and further out of control, trouble in paradise for M/L, Michael and Liz bonding over being unable to communicate with their SOs. Some great polar writers that started following the show's end include irish2002, Dimensia, joianoel, Ladygloria, and muse.
In the summer of 2003, PA moved to polarattraction.com and the polar mafia formed, which was a nickname created b/c of the barter system and ihatemyself's challenges which the inspiration for a lot of fic that came out in the early days of PA. The polar mafia was basically a group of writers/active polarists that were both well established and just starting out who got together and chatted about basically everything and anything. They later became the mods and admin of PA and created and continue to utilize a barter system to encourage more polar fic. I think the best thing that came out of the polar mafia (and definitely the most well known contribution of the pm), was Point of Origin, a round robin style fic that I am fairly certain was the largest collaborative effort of writers in Rosfandom.
Trends that started with the move (and have withstood the test of time) include the "epic" update. This was another Peachykin original, but the standard update for a polar fic is about 10 pages or so in word.
Polarism continues today and there are a good number of writers, both new and old, that frequent PA.
Other couples in Polarism
Kyle/Isabel
By FAR the most common "other" couple in polar fic. Popularized by Peachykin and Dimensia this couple was en vogue in polar fic before it became en vogue to like them in rosfandom overall. I think because there was/is less KH hate in polarism than in the larger fandom.
Max/Tess
It happened in polarism, especially prior to ITLAITB but it never was that common. Probably because again, a lot of polarists came to the "light" (haha) via significant distaste of the Max/Tess storyline on the show. However, this was probably the most common max pairing in polar fic (perhaps Max/Maria, but a lot of polarists like to joke and say "I would never punish [insert female character] with Max"), because most polarists would rather Max not be a part of the story, period. This has changed some recently, but Max is usually the bad guy in polar fic.
Kyle/Tess
During season 2, a lot of polarists fell for Kyle/Tess hardcore (and changed their opinion of Tess). This was not really the case in what I witnessed with other sub-sects of Rosfandom, (with the possible exception of Candies), so we had a few Rebels, lamptrimmers and Tess fans come to polarism because it was the largest UC ship and was sort of a "big tent" for people who wanted something different.
Rath/Liz
Obviously. What strikes me is that it isn't as common as you'd think, but when it's done it's done well. Ladygloria's Phoenix Rising is a good example of this.
Tess/Alex
Another ship that I think was accepted in polarism before it was talked about in the larger context of rosfandom. I honestly think it stems from there being more Tess acceptance prior to Departure and less love for Alex than in the greater roswell universe.
Liz/Khivar
There is a large Liz/Khivar polarist contingent -- Gaudy wrote the first Liz/Khivar fics I think in record, obviously Littlebit's Khivar fic is well loved, and Cinder's Pledge of Honor are examples I can think of.
Liz/Alec
Again, lots of love for Jensen/Alec in polarism. As I've never really spent a lot of time watching Dark Angel, I don't know that much about it, but Littlebit and 2x4 I know were big fans of DA.
Tess/Michael
A new trend, definitely, but I like to think it's growing. Or maybe I'm biased, because that's the way that my own roswell writing has gone. It's at least well received by people at PA, so there is that
Anyway, I hope this was informative, and if it's not appropriate I can delete it.