Harry Potter and the Mystery of Magical Accidents

I am not easily annoyed with anything Harry Potter related.  In fact, my normal tendency is to defend any issue someone else has with it; however, one thing that I have always wanted answered more thoroughly is why some objects are easily fixed with a reparo charm and others are simply accepted as broken.

A few examples of this are when Harry's glasses are broken, when Ron's wand breaks,when Harry's broom is crushed by the Whomping Willow, and when Harry switches the appearance of Potions text books.  One of our first encounters with Hermione in the first book is when she repairs Harry's glasses on the Hogwarts Express.  They have been broken in half by Dudley and repaired with a slim piece of tape.  The charm works successfully here.  In the second book, Harry and Ron crash the magic flying car into the Whomping Willow.  In the process of getting beaten down by the tree and the impact of the car, Ron's wand breaks in half.  Here it is accepted that his wand is broken.  No one attempts to fix it.  In the third book, Harry's broom blows into the Whomping Willow as he falls from the sky as a result of a dementor attack.  Again, his broom is accepted as a broken object and he has to get a new one.  In book 6, Harry repairs the Half-Blood Princes Potions textbook to look like a new copy that he received from the publishers so that he can keep the helpful instructions that are helping achieve his outstanding marks in Potions.  He also uses a charm to damage the new copy.

All of these instances result in a broken object that is either fixed right away or accepted as broken and unfixable.  For a while I thought that the reason for this could be that objects that were created with the use of magic could not be repaired; however, I would consider the Potions text book a magical object.  Almost all of the books referred to in the series have magical qualities:  the screaming books in the restricted section, the diary of Tom Riddle, and the Monster book of Monsters.  It perplexes me how the Hogwarts students know this unspoken rule about the repair of magical objects, especially since several times throughout the series I get irritated with the characters when I thought the objects could be so easily repaired to only find out later that they couldn't.