Yes, I did. Not right now but at the end of January. Although it may be hard to imagine a casual player getting burned out on World of Warcraft, something like this happened to me. After taking my main to 80 and messing around with several other 70s (yes, I am a bit of an alt-oholic), I became disenchanted with the idea of leveling another and another and another character to 80 through Northrend without a flying mount.
I don't know about the rest of the world but Blizzard's decision not to allow flying mounts until lv.77 didn't go well with me. Let's face it, the real reason they implemented this was not for people to "appreciate" the content but really to delay as many people as possible from getting to 80 quickly. I strongly believe the sole reason is financial.
There are many who get tired of content quickly. Having access to a flying mount would have probably halved their time to 80 and they would have tired of the end game quickly. Case in point: after Wrath of the Lich King was launched, there were already level 80 characters within a couple of days. A week later most dungeons had been cleared. Hardcore raiders were probably already getting bored.
The whole "we want people to appreciate the content" spiel failed as far as I'm concerned. People who wanted to get to 80 as fast as possible did just that, with or without flying. So this measure was not intended for them. Those who wanted to take their time, perhaps to finish questing in one zone before moving to the next, got to experience all the content. Once more, the lack of flying didn't do a great deal, except perhaps to frustrate those who hate long rides between locations.
Let's not forget that not every level 70 character has managed to purchase a FAST flying mount. The slow flyers are about as bad as epic ground mounts IMO. Yeah, they help you fly over obstacles but their speed is soooooo slow. Players who worked hard for the 6000G a fast flying mount costs, are asking themselves what why can't they use their hard-earned mounts. I am one of those. I grinded my butt off to get that mount and now I can't use it for 7 levels.
In WotLK the situation is... interesting. Players who managed to get a fast flying mount before the expansion did get the chance to use it a bit and it was great. But fresh 70s will find themselves in the situation where it's pointless to spend all that gold to get the flyer now, since they can't use it anyway in Northrend. So they have to go 7 more levels and only THEN will they finally get to fly. Neat.
As far as I'm concerned, I suddenly became really daunted at the idea of leveling 4 more 70s to 80 without flying (thanks Bliz but I've already experienced that content once), plus a 65 on another server.
So that's why I canceled my account. Of course, it's only temporary. As much as I love this game, there are some aspects of it that annoy me and I need time away from it once in a while. The break has been good. I've been working on a new website for WoW players, based on an idea which seemed interesting. It's called BankAlt.com and hopefully I will be able to launch it soon. In the meanwhile I also started this blog which was a thing that I wanted to do for some time now. I also got to catch up with a few other hobbies.
The call of Warcraft is strong though. It's getting stronger each day. Which is good. It means that when I go back to the game my batteries will be fully charged and I will be able to level at least 1 more alt to 80 before the lack of flying mounts starts annoying me again :)