First off . . . yes, there IS Evil in Warcraft, and in World of Warcraft. It is not, however, this arbitrary split of the player character races, such as there are in the Everquest games. The two factions (really almost 3) are just factions, just different sides of the same die.
Let's examine the races:
Directly from Blizzard's own web site, a nutshell history of the Orcs:
"Born on the hellish world of Draenor, the Orcs were brought into the kingdom of Stormwind through the dimensional gateway known as the Dark Portal and forced to wage war on the Humans. Although few are aware of their history, the Orcs once cultivated a noble, Shamanistic society on the world of Draenor. Tragically, the proud Orc clans were corrupted by the Burning Legion and used as pawns in the Legion's invasion of Azeroth. The Orcs managed to rebel, however, and they were ultimately able to help turn the tide against their demon masters. Led by the young Warchief, Thrall, the Orcs have reclaimed their strength and honor. Now, the Orcs stand ready to fight not for the sake of conquest, but for their right to survive in their adopted world.
Think about that for a minute. The orcs were shoved through a portal into Azeroth, against their will, and forced to ignore their noble Shamanistic ways to wage war against the Humans. They were under demonic influence, being forced to fight. Some of them, notably Thrall, Grom Hellscream, and a few others, fought off the demonic bloodlust, rebelled against the demons, and they played a crucial role in helping the Alliance to fight off the Burning Legion's attempt to take Mount Hyjal, and corrupt the World Tree, Nordassil. They forged an treaty with the "Alliance" and then took to the barren, harsh, unwanted portions of Kalimdor to try to rebuild a new society, reclaiming their noble Shamanistic heritage.
Yeah, real evil there.
Wild Jungle Trolls are mean and nasty critters, barbarous and ready to worship any sort of animal god that impresses them and seems handy. Snakes, Spiders, whatever, they're down with that, and let's cut up anyone else and eat them.
Except for the Darkspear tribe, which Thrall saved from their conflict with the Murlocs, and befriended. These trolls have shown fierce loyalty, and are prospering in their friendship with the Orcs and Taurens. Basically, the orcs, under Thrall, made friends with a primitive and brutal race, and are helping them advance culturally. Yet another noble deed of the Orcs, whereas the humans would probably just merrily slay the trolls, rather than trying to help them.
Next, the Taurens. Also taken directly from Blizzard, the Taurens "live to serve nature and maintain the balance between the wild things of the land and the restless spirit of the elements. Despite their enormous size and brute strength, the remarkably peaceful Tauren cultivate a quiet, tribal society. However, when roused by conflict, Tauren are implacable enemies who will use every ounce of their strength to smash their enemies under hoof. Under the leadership of their ancient chief, Cairne Bloodhoof, the Tauren allied themselves with the Orcs during the invasion of the Burning Legion. The two races have remained steadfast allies ever since. Like the Orcs, the Tauren struggle to retain their sense of tradition and noble identity."
Those of you who played through Warcraft III might remember, the Taurens were being assaulted by the Centaurs, and the orcs came to their aid and befriended them. Here, another peaceful, tribal society, dedicated to serving nature . . . called "evil" by people who've bought into the Alliance Propaganda.
Ahh yes, the Undead, the Forsaken. They're surely evil, through and through, right? Well, let's look at that . . . They were risen from the dead, interrupting their peaceful slumber, and enslaved to the will of the Lich King (and later some of them to Arthas - note that name, I'll come to him in a bit). After some of them finally threw off the shackles of control exerted upon them by Arthas and the Lich King, they fight to retain their own free will (rather than being used like things as so many have in the past) and simply to destroy their enemies - chief among them, the Lich King. Their closest roots are to Humans - who call them monsters and abominations, and try to kill them on sight, rather than welcoming their undead brethren back into the fold and trying to get along peacefully. They have to combat the Scourge, undead still under the sway of the Lich King's Will, for the betterment of the entire world, and are assaulted on all sides and plagued by hatred from their own descendents and family.
Some of the notable figures in undead history - Kel'Thuzad . . . a HUMAN Necromancer, who enslaved undead and rose them from their final sleep, swore allegiance first to the Lich King, then to Arthas in his quest for power (whatever's convenient, right?), and even staged his own death so he could arise as an Arch-Lich, to wield even more power.
Arthas - a HUMAN . . . not just a human, but a Prince, the King's son, and a Paladin, who betrayed humanity, slaughtered his father, slaughtered a respected and mighty Dwarf who had called him friend in his quest for power, enslaved even more Undead to his will, betray humanity and spread Undeath. Human, Check. Major Betrayal, Check. We see a pattern here? Yet, everyone blamed the Forsaken for all of this. Yeah, all their fault. They WANTED to be used and enslaved to others' wills. Now, since everyone pretty much seems to hate them, they just want to be left alone, to pursue their war against the Lich King and his evident evil.
Now, on to the Alliance races . . .
Basically, they've got the attitude of "We're still here, we've been through two invasions, and we've made alliances of convenience to survive, so we deserve everything we want just for surviving." Look back at the storylines and missions in Warcraft III. They met the Night Elves, pissed off the Night Elves, then when they HAD to, made friends with the Night Elves (not that pissing off the Night Elves is all that hard, to be truthful). They claim to be the bastions of good and light, but have a laundry list of betrayals from the human race. Not being enslaved by demons or powerful magics . . . they turned against their own kind because they wanted to, or because it was convenient, or merely a step along the road in a blinding lust for power.
Dwarves have pretty much kept to themselves, except for their alliance with Humans. Why do they ally with the Humans? Because they like to fight, plain and simple. Dwarves like to drink and fight, and the Humans are always stirring things up. They've changed recently, as they've learned that they may have been created by the Titans, and are delving into their past to try to learn more. Obsessed now with archaeology and anthropology, they basically just want to find out who they are and what they came from. They're like rednecks who've become obsessed with geneology.
Blizzard says it themselves: "it's a wonder that any Gnomes have survived to proliferate." Left to themselves, the Gnomes would have done the world a favor and killed themselves off by now. The gnomes would have you believe that "an ancient, barbaric menace had risen from the bowels of the earth and invaded Gnomeregan," but anyone with a lick of intelligence knows better. Whatever invaded Gnomeregan, the Gnomes stirred up in their incessant messing with things they don't have common sense enough to leave alone. Their own city lost to them, polluted and irradiated, they've had to shelter with the Dwarves, who aid the Gnomes in continuing their existence.
Oh, the Night Elves. Pretty defenders of Nature and goodness, right? Nahh. They're worse than the Gnomes. "The Night Elves' reckless use of magic drew the Burning Legion into the world and led to a catastrophic war between the two titanic races. " Here's the real root of it all. Burning Legion? Night Elves' fault. Orcs torn from Draenor, shoved through a portal and made to fight against their will by the Burning Legion? Oh, woops, wouldn't have happened if the Night Elves hadn't stirred them up. Even now that they've come back out into the world (Oh, crap, better wake up, the demons are back!) and been forced to ally themselves with other races, they still have an innate arrogance, and are "are very distrusting of the 'lesser races' of the world."
So what do we have there? The "Evil" Horde has two formerly peaceful tribal and shamanistic cultures who've shown honor and nobility, who are helping advance the Trolls into a better culture. The Orcs overthrew demonic bloodlust and control, rebelling against their former masters to drive them off and save a world that they didn't even come from. Despite not really trusting the Forsaken, they've allied with them because they clearly see a real threat of evil from the Lich King. They've found their treaties with the humans, big surprise here, being broken and they're being warred on again. Of course, dare they defend themselves, oh, they're EEEEEEvil.
Then on the other side, we've got the Humans, the masters of betrayal, who are arrogant and colonial, trying to eradicate the Horde in the name of "good" - never mind that they'd made peace with them to fight a common enemy when they had toi, but now that inconvenient peace thing is thrown right out the window. Gnomes who would probably be doing the world a big favor if they'd invent a time machine, go back in time, and kill themselves off before they pissed off whatever it is from deep in the earth with their radiation and pollution. Night Elves, who started the entire mess with mucking around with high magics, drawing the Burning Legion into the world, and almost certainly developing the magics that the Lich King eventually harnessed to enslave the Undead. The Dwarves . . . well, they're all right, really.
Actually, I begin to see why so many people want to play Alliance now. They WANT to be the bad guys.