This is the thread in which we post questions designed to help you get your creativity muscle flexing. Obviously there is no right answer to any of these, but they're here just to give you ideas. These are updated frequently.

What do they do with a pony that breaks it's leg?
What's the culture of the crystal ponies like? Why are they crystal?
What sort of curse would be difficult for Applejack?
What do the princesses actually do?
Celestia has prophetic visions (as seen in Twilight's Kingdom). What is the nature of these visions? When do they come? Why? What form do they take? How informative are they? Is that why her plans seem to suck when she probably instead knows that Twilight is the only pony who is going to win?
If the entrance exam to Celestia's School for Gifted Unicorns was to hatch a dragon egg, were there other dragons hatched by the other students? What about raised by them? Where are they? What happened to them?
Is Equestria on Earth at a different time period? Or is it a different world somewhere else altogether?
How much energy does magic require? Does it follow physics in that it requires the same amount of force as it would if it were normal? Does distance require more as if it were a like lever system? Or is it just some natural force that is innately part of the ponies.
How many more pony cities are there that have been lost or are undiscovered? Does Celestia keep track of them?
According the the Journal of the Two Sisters (booky by Hasbro), Celestia and Luna were born alicorns. If this is canon, then was there a race of alicorns that has nearly gone extinct?

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1. What do they do with a pony that breaks it's leg?
Send him to a hospital, not sure what else they'd do.
2. What's the culture of the crystal ponies like? Why are they crystal?
Their culture revolves around the aspect of love, that's a given. They also value tradition and kindness. The Crystals are, um, well I haven't though this one out much, but I imagine it has something to do with their snowy environment and living up north.
3. What sort of curse would be difficult for Applejack?
The F word, perhaps.
4. What do the princesses actually do?
Apart from serving as authority figures in their respective country, they are also given an additional rule over an element of nature/life or a value. Luna and Celestia are the former, with the sun and moon respectively, and Cadence and Twilight are the latter (love and friendship)
5. Celestia has prophetic visions (as seen in Twilight's Kingdom). What is the nature of these visions? When do they come? Why? What form do they take? How informative are they? Is that why her plans seem to suck when she probably instead knows that Twilight is the only pony who is going to win?
Since Celestia and Luna have ties to higher beings of existence (see below), they can be sent direct messages from angels and the like about what will or might occur in the near future.
6. If the entrance exam to Celestia's School for Gifted Unicorns was to hatch a dragon egg, were there other dragons hatched by the other students? What about raised by them? Where are they? What happened to them?
Tests are based on an earlier evaluation of the prospective student, usually their current skill level and how much they are willing to handle. Hatching a dragon egg is the hardest given test, and they are usually abandoned eggs from migrating dragons who came by the area, If they are unhatched via magic, they are sent to dragon mothers to be raised. Since Twilight was most likely the first to successfully hatch one, Celestia raised Spike for the first year or so while Twilight adjusted to her new school ,and afterwards gave him to her as a friend.
7. Is Equestria on Earth at a different time period? Or is it a different world somewhere else altogether?
It is our Earth in the future, after humans have either passed away, OR made it into a portal taking them to an alternate world, where they continue to thrive to this day.
8. How much energy does magic require? Does it follow physics in that it requires the same amount of force as it would if it were normal?
Does distance require more as if it were a like lever system? Or is it just some natural force that is innately part of the ponies.
It requires total concentration from the spell caster, and it is an inherent force within ponies. Distance can probably affect the outcome of a spell, but it's usually not an issue.
9. How many more pony cities are there that have been lost or are undiscovered? Does Celestia keep track of them?
Most cities are well known, with the lost or overlooked ones known to architects, world travelings, and higher authority figures of the world, such as Celestia.
10. According the the Journal of the Two Sisters (booky by Hasbro), Celestia and Luna were born alicorns. If this is canon, then was there a race of alicorns that has nearly gone extinct?
After Equestria was established, and ponies were declared the new major species to rule the Earth, angels came to help set up and create the new world. They came to Earth in the form of what we now call Alicorns, embodying peace and hope, and the magic of all 3 major pony species. Some eventually returned to heaven, but others stayed behind to help guide the ponies in the early times. Celestia and Luna were born from two such Alicorns, and were part of a handful of natural born alicorns from this transition period.

1) Take them to the hospital, of course. They're Ponies, not primitive Horses.

2) Crystal Pony culture is actually a fusion of pre-unification era Unicorn and Earth Pony cultures. Despite the name, Crystal Ponies are not made of crystal, it's just the side-effect of their natural magic.

3) A curse of compulsive gambling. Being the Element of Honesty does not do one's poker face any favors.

4) While there is a council of nobles and a citizen's council, both of which help decide what law or whatever gets passed, the Princesses have the final say in the matter and can even override the councils' decisions, although this is a power they rarely use. They also serve as the face of Equestria itself.

5) Such visions are rare and only occur when beings of great evil are about to make a public appearance.

6) The goal of the test isn't to hatch the egg, but rather it's a test of the student's resourcefulness, magical knowledge, and endurance, in fact it was a fluke that Twilight actually got a dragon egg (egg-shaped stones, painted to look like dragon eggs, are normally used, it's just that, some how, a real Dragon egg got mixed up in the batch of rocks) Tests such as this are reserved for only the most promising of applicants. Spike is the only known Dragon to have been hatched in this manner, considering Dragons are highly protective of them, even if the eggs are not their own.

7) Canon Equestria is, in my opinion, a completely differently world.

8) All beings have magic, but only a few (Alicorns and Unicorns to name just two) are actually able to manipulate it in a manner that could actually be called magic. Magic draws upon the ambient energies of the world and converts it into a form that can actually be used. For the most part, magic follows conventional physics (the further away, the larger, the heavier, the more numerous, etc. the target, the more energy, concentration, and actual physical stamina that is required for it).

9) Many cities, as well as the records of said cities, were lost to the ravages of time. As old and wise as Celestia is, even she's willing to admit that she knows only a fraction of these lost cities, the rest she has either forgotten or has never even heard of.

10) True, full-blooded Alicorns are not just a race, but a very large family of god-like beings, each one holding domain over a worldly force (The sun, the moon, the seasons, the tides, the winds, time, etc.) The matriarch of this family being the creator of the world, the All-Queen (hence why Celestia, Luna, and all other True Alicorns prefer titles such as "princess" over anything loftier.)