The Horde have a proud history of strength and honor. It's incredibly reasonable to assume that your average Horde member will celebrate this holiday with profound respect for the Forsaken. (You can respect even folks who you think might be inclined to eat your spleen.) I admit that I'm not entirely sure how respect for your allies translates into trick or treating, but I've never been entirely sure how that gets translated in real life anyway. I'm willing to overlook it. And considering how much fun Hallow's End can be, I'd expect the same of the Horde.
The orcs in particular, with their troubling history of demonic challenges, probably empathize with their undead companions. This could be a good opportunity for your orc character to roleplay more meaningfully with undead Guild members. "Tell me how it happened," your orc might ask. It could certainly be interesting roleplay for your orc to take the time explore Forsaken history, asking lorekeepers and scholars about Sylvanas and her history.
And given that Hallow's End is in their honor in the first place, it's certainly reasonable for your Undead to be a little more bouncy than usual. Maybe he's a little extra giddy, maybe a little more chatty than usual. He may take the time to invite friends over, and share his tale of his own personal separation from the Scourge's control. You could build a roleplaying event out of this storytelling. Imagine several Forsaken gathered around Undercity, sharing their melancholy personal tales over a bite to eat. Think of it as a kind of undead communion.
I don't know that the holiday is huge for Blood Elves. The Sindorei strike me as the kind to take themselves too seriously to cavort about, clawing at sugar and confections. Or, perhaps, they interpret the Forsaken's quest and success to break free as inspirational. If the dead people can find the willpower to shrug off the tyranny of the Scourge, then certainly the Sindorei should expect their own people to find the hidden reserves to deny their fel addiction.
By comparison, I think the Alliance will probably have lost any connection between Hallow's End and the history of the Forsaken. For the Alliance, the holiday is very much about the candy and partying. But there's still nuggets of roleplay in it all, special events that the knightly Alliance can honor.
Paladins, especially, have their own issue to deal with. A man once a member of their own order has been corrupted and cursed. While all player characters are eager to roll the Headless Horseman to get their loot, Paladins should be there for their own in-character reasons. The Horseman was once a paladin, and they step into the battle with a heavy, melancholy heart. (Or, for the more jaded Paladins, with a rage-filled heart, angry at the betrayal performed by one of their own.)
The Draenei, however, do seem like the kind to give respect to the Forsaken's freedom. Of all the races, the Draenei are relatively free of predations from Scourge's history in Azeroth. They'll be disgusted and enraged by the history they've heard, certainly. However, they didn't see it. That separation could allow them the necessary disassocation to still be able to give respect to the Forsaken's accomplishment.
Neither should the personal roleplay available during the holiday should not be inconsiderable. With Hallow's End's festive nature and decorations, now's a great time for your characters to invite friends over for a bite to eat. Borrow from some real-world traditions. Leave placeholders on the table for long lost friends and relatives, and share memories of your beloved friends.
Maybe other ghosts haunt the nights of Azeroth. Your characters' own personal enemies, once left dead and defeated, now join Thomas's hourly attacks on peasant towns. Following that track, it's easy to imagine that your characters will pick up their weapons and go to the defense of their people. (After all, you put them in the ground in the first place.)
And, lastly, Sylvanas. She's a hero, someone who helped pry the slaves away from their masters, and help turn them into the Forsaken. Now is the time for a pilgrimage. That's relatively easy for a Horde character of course, but the quest could be meaningful to an Alliance. If your character can see past the frivolity and sugar-rush of Hallow's End, he may decide he needs to see Sylvanas for his self. Struggling through hostile territory, he sneaks in to pay homage to the woman. (Of course, you'll have to decide in your own mind what the interaction between an Alliance character and Sylvanas may be. In the mechanics of the game, she and Varimathras are pretty guaranteed to simply kill you.)
Hallow's End offers a wide variety of opportunities for roleplay. It lasts two weeks, so even if you don't have any premade plans, now's the time to get together with both Guild members and your server community. Take advantage of the upcoming Wrath release -- there are many folks without much to do, desperately looking for something to keep them interested in the game. Now could be a great time to introduce them to roleplay.