These are not mine, they came from a forum post about it: Fun Little Easter Eggs in WoW - Forums - World of Warcraft
• Druids in Treant Form will catch on fire if they stand in a Cooking Fire.
• If you hug the panda cub near the portal to Pandaria in SW (or the one in Orgrimmar), it'll give you a drink that transforms you into a panda cub.
• If you /glare at a DK's ghoul, the ghoul will cower.
• If you /rude, /wave, or /salute a Stormwind City Guard, they'll emote back at you.
• The Pilgrim's Bounty pet the Plump Turkey will jump into a nearby cooking fire, turning into a roasted turkey.
• If you /kiss or /flirt with a warlock's succubus, hearts appear over your head and the succubus' head.
• If you /bow to a Pandaren Monk (minipet) they bow back. If you /drink emote at them, they will sit down and drink.
• If you /dance with the Moonkin Hatchling, he dances with you.
• The attackable neutral mobs in Stratholme will /dance with you.
• If you /dance with Mini Tyrael he will dance back with you! Its a really cool unique dance too.
There is plenty more out there, so if you find more let me know and I'll edit them in.