Hah, I know exactly what you mean, as you will have seen from my posting style, I never use one word where ten will do
It’s not helped that I write fiction and narrate audiobooks so I have a tendency to ramble, this said, I always think less is more in your Addon profile, more than a paragraph can lose people’s attention, for example here is Brigante’s.
“An unusually short male elf, standing only 5’5”, Brigante nonetheless has a muscular and wiry physique, clearly having lived a warlike life of some sort, his eyes have crows feet wrinkles around them, and his left one several scars around them, nonetheless, both his eyes and grin generally show a sense of natural good humour. Perhaps to compensate for his height, his hair is in a topknot, braided with both small silver arrowhead trinkets, and also smaller Dragonhawk feathers. Whilst he seems in generally good health, he usually walks with the aid of an engraved bone cane, without which he has a distinct limp in his left leg. His voice is fairly melodious, with a slightly teasing or friendly mocking tone to it."
Basically it gives people what they can see, just by looking at him, the rest of it, his history, where the scars came from, why he has a limp, what type of creature the bone for his cane came from, what the symbolism of the silver arrowhead trinkets and feathers have, well, that’s all things they would have to find out in Roleplay.
I never actually put his history on his Addon Profile, just that above. Because no one can -see- your history by looking at you. From the above, you can hazard a really good -guess- that Brigante is some form of soldier, but you don’t know for certain till you strike up a conversation. I’d avoid going over a paragraph and a half, personally. That is however just my opinion.
Which one? The Paladin, or the Goat? 
Yeah, there is, the reason is basically that my Guild is entirely based on the concept of Dragonhawk Riders, there are Alliance rival guilds who fly Gryphons and Gyro’s but clearly we are the best.
Thing with Dragonhawks is that they breathe fire, they kill at range generally. That isn’t going to satisfy a Death Knight’s need to cause pain directly, and also, I can’t imagine the Dragonhawk would be a) Comfortable with a thing that smells of death on their back, or b) able to form a deep emotional bond with a Killing machine that cannot form emotional bonds…
Demon Hunters? Well, I want to say the obvious…‘They’re Blind! You can’t be Aerial Cavalry if you can’t see where you’re going!’ But we now know that they -can- see, but just in a different way. Thing is, unless we are fighting Demons, that really isn’t going to help them much. They’d be relying upon the Dragonhawk to spot and react to mundane targets, at which point, we might as well have just brought the Dragonhawk on its own… That plus, well they have a Demon inside them! A Dragonhawk is not going to like that, any more than any normal person would.
So that’s why we decline those two types, we allow Warlocks as long as they do not summon Demons (Not that they would do much good n Aerial missions anyway
)
How accepting races are of different classes is pretty much down to the races involved. Logically people should always be wary of Death Knights, though there are some like Thassarian, who are trusted enough to be given field command of living Alliance units (At the battle of Andorhal, and later in Northrend), but generally? They are scary…
Demon Hunters are trickier. To both Kaldorei and Sin’dorei they represent bad times for their people. Illidan for the Kaldorei, and Kael’thas for the Sin’dorei.
They have -Demons- inside them. Its like trusting a person with a primed grenade in their hand who goes “No, trust me, I’ve got this firmly in my hand, it won’t go off” then they fall down some steps and the grenade goes off /Demon takes over.
Warlocks are trickier. We do see in the ‘War Crimes’ Novel, that Varian Wrynn is perfectly happy with having Alliance Warlock troops, summoning Demons and all. We know the Blood Elves are -pretty- OK with it, the Forsaken couldn’t give two hoots, and the Orcs have a kind of love/hate relationship with Warlocks.
Everyone else seems to view them as a necessary Evil.
Shadow Priests are not the same as Void Elves, and most cultures including the Blood Elves still accept them. Fun fact, There is one person in the Factions who is shown to use Shadow Priest Magic, and that is Anduin Wrynn.
A lot of it comes down to the culture.
That comes down a lot to characters, and how they react. Brigante has no problem with Darkspear Trolls (The Darkspear never warred on the Elves) will be -very- cautious around Zandalari, he has killed a fair few of them, but he loathes Amani Trolls and will do everything he can to assure their destruction.
It’s about preparing your response, a Green skinned Orc gives Brigante the “Huh, you elves are pathetic and weak” line, he just responds with, “And less than a quarter of our race managed to invade and take over your planet? And -how- many Wars have Orcs won against Elves on -this- world alone?”
You just have to figure out how much is the character being unpleasant.
I’m trying to think, but off the top of my head;
Humans and Worgen do not like Forsaken and vice versa.
Gnomes and Goblins are locked in a technological arms race.
Blood Elves and Void Elves are highly unlikely to get on well.
Light Forged Draenei and Void Elves are unlikely to get on well.
Nightborne are unlikely to get on with Kaldorei/Night Elves.
There may possibly be bad blood between the Blood Elves and Zandalari, as they did fight each other on the Isle of Thunder.
It really comes down to character motivation. Does Brigante hate Darkspear? No, he has several Darkspear Friends, does he hate Zandalari, not really, they were an enemy to be killed, he can bury the hatchet if they can, does he hate Amani. Yes. All of the Yes. War to the Knife with them! Extermination…
So it -really- is down to what your character thinks, and why. Why does Brigante hate Amani? He’s a Farstrider, he lost a lot of good friends against the Amani in the aftermath of the Troll Wars, in hideous ways, it was a savage Vietnam-esque war on both sides.
But that is all things that will come out of your backstory and history.