I think the biggest unspoken assumption and drive for wanting b-school so badly for some (or many?) applicants is that fantasy that once they're in b-school and especially after b-school - life will be so much easier, more comfortable, less stressful and where you're more in control. It's like an escape from what many feel is a career/job/life they are stuck in, or that they hate, or whatever. It's driven by this fantasy that b-school will magically solve all these insecurities and issues you have right now. No one will ever admit this to themselves, but deep down there's this assumption that "if I go to a top b-school, I can coast instead of struggle like I do now."
Truth is, for most of you, your careers and lives won't get easier - yes, many of you will make more money, take on more responsibility - but your careers (not to mention your personal lives) will be filled with more stress, complications and headaches that at times you will secretly wish you were back to being the young and carefree person you are now before you introduced all this additional crap into your life (money, responsibilities, authority/power, etc).
Finding a job does not get any easier - in fact, it'll only get harder as you gain more experience. You will be more stressed out about money because you will have a family to take care of (and even if you are single, you *will* be living a different lifestyle than you were in your early 20s).
So that's what you have in store for you post-MBA (having said that, in other ways as much as careers/life/etc are more stressful and complicated, it can also be more meaningful and fulfilling - but not for the reasons that you may think at this point in your life).
While you're in b-school, you will find that the MBA culture is essentially herd mentality. It's real easy to act like sheep (and many do), following each other, rushing blindly with the herd to whatever someone deems to be the "in" job to go for, or even the "in" place to live (when I was in b-school 10 years ago, *everyone* and I mean *everyone* in b-school wanted to live in the SF/Bay Area because that's what every other MBA wanted). Or to just follow what everyone else has.
The hardest part is being yourself - especially if being yourself goes against the herd. Which isn't surprising because b-school tends to attract risk averse people (folks who are more willing to conform than to stick out) - and this goes for *any* b-school.
You will find that time flies in b-school. You will feel like 2 years being part of a herd of buffalo - stampeding from one class to the next, from one group meeting (for class or club stuff) to the next, from one recruiting presentation to the next, from one social get-together to the next. There isn't much "alone time" or breathing room to reflect and think. It's go, go, go. And when you add in the anxiety of being 20-something and relatively conservative (not politically, but life-wise -- being more prone to accepting the social pressure of finding a mate and getting married just because you're supposed to do that by a certain age), it's hard not to succumb to the kind of desperate, clingy behavior of "I need everything yesterday because everyone else seems to be in a rush all the time with everything!" Again, like being in the middle of a herd of buffalo, unable to see where the herd is even going.
And before you know it, you'll graduate. Hopefully with a job.
Speaking of jobs.
In strong job markets, you are surrounded by MBAs who develop inflated attitudes of their self-worth - acting cocky, even dismissive of certain kinds of jobs. And expecting to walk in and change the company like a know-it-all CEO. So discussions center on schoolboy fantasies about how rich they are going to be, how they will conquer the world, and other kinds of generally obnoxious attitudes.
In bad job markets, you're surrounded by MBAs who are freaking out about getting *any* job, fearful of that huge loan they took out. So you're in a culture where everyone is on the verge of developing an ulcer, praying and pleading that they will get *a* job (and yes, this goes for all top b-schools. The world is not waiting in anticipation for an MBA's declaration that being the master of formatting Powerpoint slides equates to "leadership" (they'll just call it "thought leadership").
Also, you will realize that not everyone in b-school is a great person, regardless of what the b-schools like to say in public about how they foster a "team-oriented environment". There are a**holes in any environment, and b-school is far from being immune from that. Even in the most "cohesive" schools, it's not all kumbaya, even if on the surface everyone is touchy feely with one another (as an aside: some of the down-to-earth folks I've ever met were HBS alums; and some of the most pompous douchebags I've met were Stanford GSB folks). If you don't find any one of your classmates to be obnoxious or grating, then chances are you are completely invisible, or you are the obnoxious and grating one. There's going to be clashing personalities, and you won't be able to avoid it.
Profs: going to top schools doesn't grant you immunity from profs who can't teach. Or individual school administrators who are incompetent. Again, most people do the best they can, some are really good or exceptional, but you will encounter a few bad apples (if not more than you expect).
Finally, you will feel the true weight of the fact that it's REALLY expensive. REALLY REALLY expensive. For some people, it can be an emotional albatross. And because you gave all that up, chances are, you will develop a serious case of cognitive dissonance for a while about it being "the best time of your life" - and only really see the experience for what it is - with the benefit of hindsight.