The only questionable ones are the wolfhawk and priest mount, with a shout out to the monk mount if he can’t keep his trap shut and pretend to just be a normal tiger.
Have you seen Ban-Lu.
It doesn’t even resemble a normal tiger.
If you want a tiger mount use the Shado-Pan one or get out.
I would say the Warlock mount is also off limits. During the quests where the warlock asks about it, they get a response of " While the wyrmtongue knew little of the steed itself, it is clear that Hel’nurath has thus far only been able to create one. He has gifted it to his greatest general, the fel knight Arax’ath. The tricky part is that Arax’ath seldom leaves Xoroth, preferring to command his armies from afar."
So it seems there are VERY few wrathsteeds in existence. They are not common and are only a very recent creation.
Yes I have all the class mounts.
He looks like a (magic) tiger. It’s not as if he has “Xuen’s Kid” tattoo’d on his forehead or anything.
Sure except it just looks like a high def dreadsteed and there’s nothing unique about those.
So just use a Dreadsteed.
My dreadsteed isn’t high definition. And it can’t fly.
Maybe because Dreadsteeds are not meant to fly. It is not in their capabilities, otherwise they would.
…or they were added in vanilla, before flying was even in the game and three expansions before it was added to azeroth.
By your own argument the DK one isn’t okay because it’s a very specific dragon we raised, and the paladin one isn’t okay because it’s a very specific horse and the DH one is a very specific felbat.
Why no objection to those?

He looks like a (magic) tiger. It’s not as if he has “Xuen’s Kid” tattoo’d on his forehead or anything.
Fyte me irl.
Because for the most part, felbats are commonplace. Frost wyrms, I’ll need a bit of convincing on that one but could see multiple used. Wrathsteeds, however, are a completely new breed of flying demonic horse with very few in existence, and they are visually different.

Because for the most part, felbats are commonplace.
Not this felbat, it was a named case - a special felbat that the matron raising them specifically thought the Slayer should tame. Unique to the world, no different from the likes of the named Wolfhawks.
There’s only one Wrathsteed.
There’s only one Lixahl, the Slayer’s broken felbat.
There’s only one Kyranastrasz, the Death Knight’s risen dragon.
https://d1u5p3l4wpay3k.cloudfront.net/wowpedia/thumb/e/e6/Hellblazing_Reins_of_the_Brimstone_Wrathsteed.jpg/300px-Hellblazing_Reins_of_the_Brimstone_Wrathsteed.jpg?version=1ba4a1f25cf7ba91946ec954959c388d
https://wow.zamimg.com/uploads/screenshots/small/4449.jpg
So different!

Not this felbat, it was a named case - a special felbat that the matron raising them specifically thought the Slayer should tame. Unique to the world, no different from the likes of the named Wolfhawks.
I think you’re deliberately being nitpicky. I only raise the concern because Wrathsteeds are not felsteeds, while a felbat is just a felbat in the end, seen to be used by many Illidari. And since the class mount is the only available felbat mount in the game, people have to use it to represent their mounts. If more variants come along, they will undoubtedly be used.
In the end, this is just my opinion and why I believe some class mounts should not be used. Others have explained why some others are not being used very well. Not that I can stop anyone from doing it, but this is my personal stance on certain mounts.
maybe i don’t want to use the dreadsteed because it can’t fly.

Because for the most part, felbats are commonplace. Frost wyrms, I’ll need a bit of convincing on that one but could see multiple used. Wrathsteeds, however, are a completely new breed of flying demonic horse with very few in existence, and they are visually different
Yeah, remember how people played green orcs and force their make-believe on others by saying they are actually brownskinned Mag’har?
Now imagine if we don’t all say we are riding the Netherlord’s Wrathsteed of Xoroth IC but a highres demonic horse. Shocking and a vast logical leap, isn’t it?
Again, there is a difference between not having an alternative and being forced to use something else while portraying what you desire, and deliberately using something different for the sake of being cool.
Ever considered that you don’t deserve to fly? Eh?!

Again, there is a difference between not having an alternative and being forced to use something else while portraying what you desire, and deliberately using something different for the sake of being cool.
How are those two things mutually exclusive…?
I occasionally use the purple wrathsteed and say its a demonic horse, potentially from Xoroth, with some armor slapped on it and it has demonic wings.
Why wouldn’t it be plausible that a warlock alters their demonic minion and improve it if it’s their mount?
Priest and Monk are the only ones you won’t get away with. Wolfhawk can if you pull it off right, same as all the rest.
But the illidari one is wearing special, unique armor too, it’s completely unique, and so are all the others by the logic you’re using. What makes you say that the warlock one is offlimits and all the other ones aren’t? Who says illidari NEED a felbat? Who says DKs NEED their frost wyrm? And so on.
None of them are necessity, all of them are preference. Warlock the same as any other. The only unique ones are the monk, priest and hunter ones, because those are explicitly very unique things and not just ‘a demonic horse with a bit of armor’.
Bonus round:
Paladin could use a normal horse
Druid could use normal flight form
DK could use one of the myriad of other flying undead mounts, dragon or not
Demon Hunter could use one of the myriad of other flying Fel-beast mounts
Warrior could use the ‘normal’ proto drakes
Why would a mage need a mount IC? They can just teleport
Rogue could use any other bird IC
And so on.
To clarify: My point here is that the ‘b-but no alternative :(’ point is very unbased, uncool and about as muscular as a Nightfallen.

Why wouldn’t it be plausible that a warlock alters their demonic minion and improve it if it’s their mount?
Changing a creature would require an extreme amount of power. Lord Hel’Nurath was only able to create one Wrathsteed, so it seems a bit off that suddenly warlocks everywhere can do the same feat in what appears to be a far shorter amount of time if Hel’Nurath had been working on it since his defeat in Vanilla. I am not saying that people should be ignored for doing it. I am just putting forth my reasons as to why I find it questionable and won’t do it myself.