I'll elaborate a bit on this, as I should of.

Take someone out of their comfort zone in this game and suddenly things are alarmingly difficult to them, case in point the drakes of Occulus mentioned above. No matter how brief the encounters or how little of the content focuses on vehicles (admittedly, the amount is increasing throughout the game to the point that it's getting a tad overdone, but not so much in instances or raids that it's worth crying about for the 'hardcore' crowd) people seem dead set against vehicles as a means of variety.

Yeah, yeah, you're not controlling the character you leveled to 80. Last I checked, variety in a video game is a good thing.

Occulus is fine, (probably because I've only ran it a handfull of times) but the seige event in Ulduar is rapidly growing stale.

Anyway, vehicles or not, it's not variety if it's the same crap week after week.

Ulduar siege vehicles growing stale? Wow, no way! After five weeks now, I still love it. And guildies experiencing it for the first time also love it. I wish there were more vehicle bosses/encounters of increasing difficulty, including aerial assaults. Optional bosses, of course, but still more.

Now, if I could just convince everyone to start leaving some towers up *sigh*

I'd thing that this is less of a gimmick, and more of an immersion tool. Vehicles have always been a big part of the Warcraft RTS games. Assaulting a city with an actual siege engine makes a whole lot more sense than being on foot and stabbing a wall.

Gimmicks arn't always bad for RPG's, so if they are done right they could be good for MMORPG's as well. How many people here spent hours and hours breeding chocobo's in FF7? At the end of that time you got a shiny new spell.
It works the same way for WoW, after mastering the vehicle "gimmick", you can get some shiny new loot from FL.

If I wanted variety I'd make an alt, like I did, twice. What I don't want is all the effort I've put into learning my class and how to play effectively reduced to nothing and having to use a poorly conceived vehicle instead. Its boring and obnoxious.