Do you think players carried on the mentality of belonging to a fraction or they were just restricted so it was kind of a gameplay thing. I only know a few people as of now that are Fraction specific.
I was horde all the way until myself and a couple of friends got sick of every time we played PvE together consisting of one or more getting boosted/carried, because we were all different levels, and one friend didn't play much. Our solution was to roll characters on the other faction so we couldn't trade stuff from alts, and only play them when we were doing content together (as in, in the same room). We did a bunch of long story quest chains (the ones that reward a blue item), and ran every dungeon up until Uldaman as a party of 3 (paladin tank, druid heals, hunter dps), by which point we were finding the content way too easy, even though that's a dungeon with level 40 bosses, and we were around 33. I remember healing the last 3rd of the dungeon naked (no gear equipped), in cat form, and blind drunk IRL. After a year of 19 twinking together, we were just running circles around PvE content.
I realised that there was a whole other side to the story of the game, which explained a lot of what I'd seen while playing horde. Now I think of playing only one faction a bit like watching a TV series, but skipping every other episode.
And then you have the kind of people who take having a 'main' so seriously they never really play any other classes:
https://www.youtube.com/watch?v=0odasQlUmEA
When I started twinking at 19, almost everyone started with a hunter or rogue (easy to gear and low skill floor), and we'd make that our 'main', but it soon goot boring running round pre-xp-lock BGs one shotting all the level 10-12 cloth wearers (and getting the nickname 'Hoover' after the vacuum cleaner). There was one popular horde player in the guild, Roguer, who rolled an alliance mage, and right from the outset he was impossible to beat for any of us horde rogues. We didn't swap factions, be we did roll cloth classes ourselves, and put all our past experience playing as hunters and rogues to use, now playing as their usual targets against them, while knowing exactly how hunters and rogues played, so we were one step ahead.
Doing that I realised that focusing on just one class in PvP puts you at a disadvantage, because the best way to beat an opponent of a different class, is to understand how they play it.
Of course, I've had more time than most to put into levelling and gearing a ton of alts.