I would be tempted to say, that if they merged servers and I lost 2 or more names, I may be so grumpy to stop for a bit. Though knowing myself, I would keep writing to Blizzard to get my names back. And I refuse to use special charecters just to have my names.
World of warcraft has given me so much, not only gameplay, but also social. So, I can't see myself leaving for something changing in the game, I just see possibilities.
Like OP said, he/she would leave if Sylvanas was killed off. I see it as a chance for a little bigger war/RP/gameplay. If Sylvanas suddenly falls, then I imagin Gilneas, Stormwind and maybe even the last people of Lordaeron, trying to assult and reclaim their lands - making Warcraft a war again. But that's just one view point. Maybe would lead to the forsaken have to adept, or finding a new home. Or my biggest nerd scream, they are sealed within the Undercity. Alliance building and reclaiming the lands above, as the forsaken a captured below - leading to a squad of goblin tunnel diggers to breach into Undercity with a digging machine, making a new entrance that leads out a little further away from topside. Giving a little feel of a war of need, creating terror as you would see Alliance soldiers attempting to hold back rampaging forsaken who tries to reclaim the top side.