Onyxia is pretty far from Vanilla experiemce, or am I forgetting something?

Just started playing it as Human, and already noticed mobs spawn very fast, and gathering-related materials probably do too.
Mining takes only 1 try to collect all minerals/rocks, and it's impossible to fail it too.
HP recovery is way faster (it should start slowing down around level 6, if I remember correctly).
Heroic Strike ability has wrong Hit Impact color (should be yellow and red, not purple).
Mounts are obtainable at Level 30(?) instead of 40.

Is there any other significant thing that I am missing (besides higher XP rates and Dual Spec which people talked about already)? And did I get something wrong?

its never announced as 1:1 vanilla realm.
its a seasonal realm based on frostmourne where u able to experience the contents of each expansions with QoL additions like dungeon finder and things u have mentioned.

And there is no real 1:1 Vanilla server that has at least 2K of population, correct?

Lordaeron is the closest one to vanilla, but not entirely 1:1 and its 3.3.5a wotlk.

Onyxia is on Burning Crusade content, the Vanilla stage ended months ago.

oh i thought the guy was referring the term as in original or without custom 'vanilla', assuming hes already read the realm info panel thingy on the character selection.

I'm just going by how people call the content, the original release of the game without any expansions being "Vanilla" WoW.
That's what's been used long before anyone decided to hand out diapers with Classic.

As mentioned Onyxia is on Burning Crusadenow. Having said that some of your points are correct. But it should be noted that Onyxia works by hacking (for your benefit) the 3.3.5 client to accept the older versions of spells etc. There are likely limits to this technique.

The mining point is correct. Mining only requiring a single hit was a change made in the Wrath of the Lich King patch 3.0.8. However it may well be a limitation of what can be done with the client, or the developers didn't see this as an important enough aspect to spend the required time to make a good ROI on that time.

Mob respawn rates, well. I cannot remember when the dynamic/group based spawning was added to the retail game for mobs (it was always used for node spawns I believe). But for sure, I do remember that burning crusade didn't seem to be much faster than the later patches of classic vanilla. This though, is a general thing with private servers, mostly a legacy result of having larger than normal populations and originally not having a dynamic respawn rate as an option in the server core. Done properly, you can have long respawn times but have a minimum number of a group of creatures/nodes up at any time to ensure a level of scarcity when it is busy, but always have some available. But, I've not yet seen it done in the private server space, and even Blizzard on their "classic" servers seem to have done it badly. So, I'd personally not make the complaint. But in terms of if you wanted to compare original Burning Crusade it's probably a valid point. But, I would say it's a valid point for all current versions on all known private servers out there.

Health regen.. Cannot comment, I've not leveled on Onyxia myself in BC. I would guess maybe it's at the same settings for 3.3.5, and again it wasn't deemed so important enough to spend time to implement.

The colour of an effect is unlikely something that warmane could fix UNLESS there's already a spell effect in the 3.3.5 client they could re-use. So, that's a tough luck situation I'd think. Also even if they can send new client side spell effects using the hack trick they use. Again seems like something that would take some time to do for little return. The vast majority of players wouldn't notice this.

Mounts being attainable at level 30 is correct for Burning crusade (last patch) mind you. I would expect during the vanilla phase it was correctly 40.

Really a lot will come down to the fact that the client is 3.3.5 and is being adapted by the server, and there will be limitations here. This along with the fact that the server core is primarily Wrath of the Lich King 3.3.5 and they've changed it to match Vanilla and Burning crusade. There will be things that a) They didn't know about/needed more info to get correct values for and b) Just cannot be done even with the way they can hack the older spells etc into the client and c) That would just take more time to implement than it is worth in terms of the immersion breaking effect.

That's what I think at least.

^ Thank You all. And I appreciate your thourough answer, r00tk1t