EW started standard enough; the two starting fights were one technical dance (Hydaelyn, this time) and one free-form fight (Zodiark), but they were...different. Hydaelyn was a lot harder for people to figure out on sight since "hiding behind cyrstals" wasn't something non-Savage players were at all familiar with. Generally, things on the field don't do much unless the players have control over placement (such as the rocks before Behemoth uses Meteor in Labyrinth of the Ancients). I remember many of my first runs, the entire party being completely confused why they were dying in those sections. Hydaelyn is also teleporting around quite a bit and there is a lot of running around by the party, including the two most wipe causing mechanics after the first Lightwave phase, Chackrams being a big offender (6.0 Expedience was amazing for this, though), and the final phase doing two sets of stack/spread during Lightwaves. And this was the more technical of the fights, which is typically the one with less movement.
Conversely, Zodiark...was Danger Dorito. A lot of people had trouble with the rotation mechanics, particularly the Snakes, but if you had a good Dorito, everyone could just hug their butt for the entire fight (other than the tankbuster/swap mechanic) and be fine. For the less technical fight, it required a relatively low amount of movement and pretty much zero coordination, which is why it was solo healed (and solo tanked) extremely early in the expansion's lifecycle.
But things just got weirder from there.
Patch 6.1 saw the introduction of the time traveling, multi-ring head insanity that seemed to catch a lot of people off guard. While HadEx and SeyuEx were both pretty challenging, the pace of the attacks along with the mechanics for this one was again a step up that seemed to throw a lot of people off, and the mechanics again seemed more confusing than anything, with guide creators writing various "cheat sheet" ways to remember that the ring arrangements meant and how they should be handled. Something we'll see again in Ex5. Overall, I only ran this a few times because...well, no one really WANTED to run it much after getting clears.
BarbEx was completely different again. While RubyEx had a fair amount of movement, the mechanics were mostly straightforward and there were large periods of time to attack the boss, as well as a checkpoint (which Emerald would also inherit). But BarbEx was different. The damage spikes and mechanics being thrown out were high pace and high intensity compared to the norms for Extremes. It was more akin to a Savage fight, and consequently, Savage raiders loved it, but it seemed far less methodical or technical than Extremes typically are, and had several points of failure and body checks, despite having a somewhat lax Enrage. It ultimately wasn't so much hard as it was an endurance fight of just dealing with a constant wave of unending attacks for 10 minutes straight with a few transitions to briefly catch your breath while also not being able to do anything.
Then we have RubiEx. This fight is just weird in every way possible. There are EXTREMELY long periods of downtime with no unavoidable damage coming out, yet the damage that does come out is often lethal if you DON'T avoid it. The maze game is much less understandable than things like Suzaku's field mechanic, which by contrast is super easy to identify and understand, and combines several annoying things together: (1) it's a slow mechanic in one way, since it takes so long to happen and the boss is doing LITERALLY NOTHING ELSE while it's happening, (2) it's extremely fast once it DOES happen, giving you little time once the platforms are actually all in place to determine the safe spot (again leading to guide creator "cheat sheets" like the M trick), (3) it repeats the rotating weirdness that seemed to throw people so hard in ZodEx, (4) but because so much of his fight is this one mechanic, much of the REST of the fight seems somewhat boring by comparison.
His only other big mechanic is all back-loaded where he does the expanding + attack (which doesn't even look like a plus once it opens into a box, throwing people off on trying to identify the safe spots as there's no kind of telegraph or normal mode equivalent) combined with stack and spread (which are only indicated by small debuffs -again, normal for Savage fights, but a-typical in Extremes which normally have more obvious markers on party members like BarbEx having the stack on a party member and 2x flare markers on the tanks), followed by a alternating stack or spread mechanic which is ALSO paired with the boss doing one of two attack patterns depending on a tell on his model (also something typical of Savage fights but less so of Extremes, though this isn't AS unheard of, like Titania's Rune attack, though in that case she always does one then the other so you always know what at least one of them will be), which he follows up with a body check mechanic (now that everyone's dead - hope you have a Healer up and saved the LB3..!) combination Limit Cut DOUBLE tether swap. As far as I'm aware, this is the most difficult version of Limit Cut in the game's history, but definitely the most difficult one ever in an Extreme, and it comes right after a body check.
Collectively, during this series of mechanics, RubiEx does the vast majority of the ENTIRE FIGHT'S damage and pass/fail mechanic checks, after about 6-7 minutes of standing around with a thumb so far up his butt that this fight has already been solo-healed WITH NO TANKS because a DPSer can "tank" his general mechanics as his damage output against his primary target is so low to render a Tank meaningless, and unlike BarbEx or even ZodEx doesn't seem to have any mechanics forcing a Tank swap or high mitigation.
One of the distinctions in the past between Savages and Extremes was that Savages generally require people to die a bunch to piece together the mechanics while Extremes are more intuitive, and that Savages are more about proper and precise execution of the dance while meeting Enrage DPS requirements while Extremes are more about execution of slower paced, more understandable mechanics. While Savages often require twich reflexes and precise positioning, Extremes tend to be more technical and forgiving, and while Savages have long had mechanics that require full party coordination and single point failures (one person messing up likely causing a wipe and two messing up guaranteeing one), Extremes often have it where if you mess up, it kills you but GENERALLY not the entire party, with a few exceptions.
Extremes also tend to have smoother damage curves on the party, with less damage stacked all at once outside of some specific mechanic (Miztek often calls those the "whombo combo" of the fight, like the multiple tree attacks in Titania) that generally consist of 3 or so mechanics going off together, while Savages have this more frequently through fights in general.
But these Extremes often have multiple "whombo combos" throughout them, sometimes repeated several times, and even the non-combo mechanics often kill players and wipe parties, sometimes with and sometimes without these odd "dead" periods between them when nothing is happening. Further, the "body check" point of failure and collective party coordination has edged into Extremes. One can argue why this would be good (teamwork makes the dream work, and it's an MMO!) or bad (many people who don't have Statics run Extremes as their endgame - they don't have Statics, so they don't have that level of coordination accessible to them as they're dependent on what PF has "blessed" them with that day), but it IS a change. The pace of mechanics is either higher (all except Ex5) OR the mechanic sets up slowly but then executes all at once rapidly/faster (Ex5 in particular).
Finally, the biggest change seems to be that the mechanics are far less visually apparent/intuitive - something that USED to be a big distinction between Extremes and Savages. When the party needs to stack to catch the thrown tank in Byakko, there's a very clear stack marker. When the party needs to stack in RubiEx, there's not, just a small debuff on one player and small spread marker debuffs on 4 others.