Anyway, I thought I'd add my "why I hated school as a gifted child" list to this thread: This applies to public and private schools, both of which I've attended. This is an NYC-based, personal, anecdotal perspective, and doesn't feature any sort of statistical rigor.
* Co-student apathy, class disruptiveness, and disinterest. Seeing other students cheat, fall asleep, space out, not participate, and fail horribly can be a motivation killer.
* A predilection toward violence by many students/the constant threat of getting "jumped" (less of an issue at private, but not completely eliminated)
* Overcrowded classrooms
* Teaching to the lowest common denominator. No attempt at tailoring education to individuals or logical groups.
* At times, more time spent on attempting to restore order than teaching
* A habit of blaming "everyone involved" (i.e., anyone in the vicinity) instead of attempting to investigate and prosecute the perpetrators of activities that were against regulations. A very "short-cutty" attitude towards determining any sort of blame. Nothing resembling due process and no requirements for sound evidence. "10 guilty men go free" had no meaning.
* Little to no coverage of the source theories or practical future applications of subjects. Much focus on facts, figures, and formulas without any attempt at relating it to the real world, past, present, or future. "Real world" examples tended to be laughably contrived. Attempting to glean information about sources/reasoning for the thing's existence often met with angry stares ("you're causing us to veer off the subject and confusing people!") or simply stopped cold ("it doesn't matter, just learn it").
* Poor facilities and equipment (not an issue at private)
* Alternately attempting to paint the currently taught education as a personal enrichment experience ("it's for your own good") or a necessity for future survival ("learn this or you'll sleep in the street"). Neither perspective was accompanied by anything resembling why or how it was beneficial.
* A jarring, cacophonous, impersonal, zoo-like atmosphere. Getting up early, waiting outside in the cold/heat, being stuffed into classrooms/buses/lunchrooms with people who had horrible hygiene; disgusting habits; were loud and obnoxious; were dramatic, crying, whining, moping children; were four feet tall and no life experience, yet filled with endless arrogance and bravado. Blaring bells going off when you're still half asleep. If you were precocious/mature, you tended to feel very uncomfortable and out of place. Arguments about this being like the "real world" are complete and utter nonsense; if I don't like a place of work or other institution, I go somewhere else that I like better.
* Absolutely horrible food (even at private school!)
* Far too much focus on testing rather than the ability to understand and apply knowledge (which goes right along with not attempting to teach how said knowledge could be usefully applied).
* Lack of depth. I've learned (and continue to learn) more about subjects researching them on my own than I ever learned about them in school. Perhaps that's a given considering how much time we can personally spend on subjects outside of school, it still seems like schools could do a lot better job of it.