Magick and spell casting can certainly be used to achive many great and wonderful things, but they too of course have their limits.
I have read various forum comments from people desperatly trying to turn into a werewolf or vampire, or turn completly invisible. This website, although full of useful information and knowledgable people, is also filled with various fluff. Here are some limits in magick so you may be able to seperate the fluff from fact and put your energy to an achievable goal.
1. Changing Your DNA/Apperance
It is completly impossible to transform into some sort of supernatural being or to change your physical apperance. The only way I can imagine for you to change your apperance would be for you to affect the minds of others into 'thinking' that your apperance is different. Even that I doubt.
Shapeshifting in general is an impossible feat inside your physical body. It is not inside your astral body however, and this is often what confuses people.
It is impossible to become invisible; however there are spells to make you very unnoticable.
2.Changing the Laws of Physics
It is also impossible to change the laws of physics. This rule is something of debate, as there are some who believe in levitation and telekinisis.
3. Creating/Destroying Energy
This applies to science as well as magick. You can draw up energy, or send it away or change it, but not create or destroy it.
4. Answers to all your problems
If you were to break your leg, magick cannot heal it instantly on the spot. If a building is burning down, you cannot use magick to stop it (there are some who believe in the power of psychokinisis who would say otherwise). Often, the simplist answer is achieved WITHOUT magick.
These are the major limits for magick that I can think of. Please add any that you think I missed. Blessd be!