I've spent the last few days leveling up my pacifist rogue...
For roleplaying purposes, I figured a rogue was the closest Warcraft class to a spy, so after some googling for pacifist spies, I found Noor Inayat Khan, daughter of a pacifist Indian Sufi Muslim and American mother who was raised in India, France, and England, and who was a radio operator in the French Resistance for the British during WWII until she was captured and executed by the gestapo. So here is Noorkhan.
It's going a LOT better than my undead priest.
First, there have been no XP plateaus - I've gone up to level 11.5 without having to explore for XP, and I haven't explored outside my "appropriate" level (which loses some XP due to capping) with the small exceptions of seeing Menethil Harbor and Auberdine. Alliance seems to have a lot more quests, and a lot more quests that can be completed without combat (gathering, delivery, cooking quests with normal drop ingredients (doable via auction house), talk-to-that-guy quests, etc).
Second, a gnome rogue is probably the best pacifist character:
1) Money is tight before level 5 and a gathering skill can be learned, but a rogue learns pickpocketing at level 4, and the non-aggressive Troggs can be picked for primitive rock tools that vendor for 22c, a lot for a level 4.
2) Target dummies can be made by a gnome engineer at level 5 instead of 10. I was able to do the Emerald Dreamcatcher quest at level 6 or so.
3) I can stealth while getting exploring XP; especially handy when I explore Horde areas.
4) There may be some quests where only a rogue can get "drops" by pickpocketing instead of killing mobs. I was able to partially do the Shimmerweed quest via pickpocketing, but since players can also get Shimmerweed by opening the baskets, non-rogues can also do this quest nonviolently.
One minor annoyance: after hitting level 10 and getting Sap, I tried it, and it made my dagger skill go up! So I don't have a perfect "1" in my dagger skill... There may be some quests where I need Sap (in order to stay out of combat), but I'll try to use it only when I absolutely have to.