Xavius...

The final boss of the first raid, who has taunted and menaced us throughout Val'sharah, has been considered quite undertuned by most people.

I wondered if this opinion was based on the mythic 'race' to world first and didn't really apply to us mere mortal raiders.
I'm not even convinced it applies to Exorsus et al who approached this boss with unprecedented gear levels for early expansion content.
We are used to seeing the world first race fought in dungeon blues with a smattering of raid gear, this time the winners were fully decked out in mythic+ gear better than heroic raid level!

If the current gearing model continues, this could spell the end of the world first race as we know it, with the number of Mythic keystones the top guilds can lay their hands on being more important than raid performance.
Even now, a lot of the wonder has gone for me. I used to be astounded that they could produce such amazing results from such meagre gear, I used to know those players were just flat out better than me! Now though, it's all about the gear... honest :-P

World first races aside, the rest of us are just plodding along at our own pace.
So how did you find Xavius?
I still haven't worked out what the whole grey room thing is about, I really should pay more attention. Nobbel has a Xavius lore video out so maybe that will shed some light on it.

The main mechanic (gimmick?) of the fight is Corruption. It's mostly your typical 'don't stand in the bad stuff' check but it has some really nice aspects that set it apart with further mechanics being introduced as your corruption rises.
At 33% giant corruptors appear at the edges of the room and occasionally throw out balls of corruption which cover about a third of the floor.
At 66% there are even more adds which throw smaller balls of corruption but much more frequently and they can land anyway, often making you dance to stay alive.
At 100% corruption you briefly become more powerful until you succumb to the corruption and it controls you, forcing your friends to kill you.

I particularly like how it affects each individual separately rather than the whole raid, you can't even see the mechanics affecting the other people until 100% corruption, and how this then plays out with the other positional mechanics.

On top of the accidental corruption, there are some mechanics to deal with that are guaranteed to give you corruption. 
This is where the 'dream' state comes in...
Half the raid are pulled into the dream state which has no effect that I can tell apart from resetting your corruption when you wake up and return to the real (nightmare?) world.
It has the nice bonus of resetting your cooldowns too.

All corruption mechanics should therefore be dealt with by these people as it will be reset, and this makes the fight much more interesting.  Reaching 100% corruption in the dream state even gives you the power boost and you return to the real world instead of being mind-controlled when it ends.
This is much more difficult than it sounds though, I've tried to reach 100% in phase 1 but there doesn't seem to be enough corruption to go around!

Phase 2 is probably the most interesting but the hardest to deal with as a shaman.  It's the old weaknesses coming to the fore again... low health adds and high movement. I'm always tempted to ignore the adds, they die so quickly that it hardly seems worth switching to them and some classes are just so much better suited to killing them than the elemental shaman.

Our raid leader rages about switching to the adds all through the phase. They die almost instantly so it's never an issue, soaking the pools they leave behind is the only issue that really comes up, but the raging continues. 
It's quite amusing to be honest :-)

Phase 3 is just a straight sprint to the finish line. Can you kill him before he kills you or too many people are mind-controlled?
For some reason, one of our healers is always mind-controlled quite early.  It's always the same guy, no idea what he's doing wrong but his corruption just rises so much faster than everyone else's.

We killed Xavius on heroic for the first time on Monday then repeated it on Wednesday as part of a full heroic clear. 
Our first kill... Xavius Heroic

How does Xavius compare to other 'final' bosses?
Visually, he is far superior.  Even in a grey, featureless room, the fight looks fantastic!  It is almost the antithesis of the grand arenas from MoP's Mogu'shan Vaults but it uses that bland background as a blank canvas.
Demons from your nightmares slowly march from the fog, while others stay on the periphery as ghostly images to assault you from afar.
It's all done superbly and adds to the nightmare feel of the instance.

While it appears to draw mechanics from some past bosses, corruption/mind control from Cho'gall in BoT being the most obvious but not the only one, there is enough of a twist to keep them fresh.

Unfortunately, it doesn't offer the same challenge as any final bosses that spring to mind...
Even if we just stick to the first raid from each expansion, how does the challenge measure up?
Highmaul - Imperator Margok?  A gruelling fight
Mogu'shan Vaults - Will of the Emperor?  Arguably similar but we also had to contend with Empress Shek'zeer and Sha of Fear.
Cataclysm - Cho'gall, Al'Akir and Nefarian?  Not even close.
You might have to go all the way back to Kel'thuzad in Naxx'ramas for a comparable challenge.

Am I judging it too harshly though? 
Are we over-gearing it much as Exorsus over-geared the mythic version? 
Am I just better now than when I failed all those many, many times on Nefarian?
Probably yes to all three.

Bring on the Mythic!