Firstly, what separates a good practitioner from a great practitioner in magic? Is it subtle details in their incantations, their preparation and execution of rituals, or is it something more personal? Does the universe consider emotional conditions, integrity of a cause behind spellcasting, ect.? Is magic more powerful depending on the aura of a person? Are some people just born more lucky for their magics to be more effective and consistent?
Secondly, how are spells created? For one to understand the craft of making spells, that means that magic should, for all intents and purposes, have a methodology behind how it works. It's own set of mechanics, otherwise anyone could create any spell with any ridiculous incantation or ritual and it would be equally as valid as any other. I've looked a bit at these forums, and it seems that some senior members concede that not all spells found on this site are bound to work.
In my opinion some people have a knack for magic, others have to work harder to be able to develop certain skills. I would say there is no standard which you can compare two practitioners. Even within traditions, a practitioner's work is their own, and how one person treads the path is not necessarily how another one does.
What makes a great practitioner? Dedication to their practice, sincerity, fluidity in their mind, and perhaps a desire for knowledge; be that of the self, or the universe.
Auras, emotional conditions, luck. All of these are individual differences and as such would depend on the individual. Anger effects me differently than it would someone else. I keep a magical diary in which I record ll possible extraneous variables in a working- over time you begin to see patterns. But everyone is different, I find I get better results at night, others during the day. Summer is the worst season for me, for others it is the best.
Focus on yourself, your own inner mechanisms and your own work and resist the temptation to compare yourself to others.
Unless you belong to a tradition, everyone will have their unique method of casting spells. Even if you belong to a tradition the techniques employed will often have an individual twist. I personally think people who follow other's instructions to the letters are unimaginative.
Everyone does their magic differently. I think you need to understand what magic is and understand that there are certain skills employed within it. This includes such things as- calling in energy (general invocation), calling in a specific energy (specific invocation), focusing the mind and the entirety of the being, visualisation, and many more skills which various books will walk you through.
All changes you wish to cause must already be able to occur within nature. As I have said, however, the means to cause this change are many, and whilst certain skills may be universal, how you practice magic is a decision only you can ultimately make.
Magic is an ability just like any other ability. Some are better at it naturally, some learn it like any other ability, some pretend to know it. Magic to me is actually something everyone can do to varying degrees because I believe it is within the grasp of every human (so not that magical in itself), just that some don't know this, or refuse to believe it, or have a warped sense of what it should be so they give it characteristics that make it seem that a person is doing something beyond regular human grasp.
Most folks gain their knowledge from consistent practice, trial and error and copious amounts dedicated time. To be a great practitioner, you do whatever it takes to be great at something. It's just like studying at school except that there may not be a set curriculum, only general guidelines and you need to discipline yourself enough to sit down and get working.
Regarding the spell creation process - it is not so much the spell as it is the ability of the spell caster to properly place intent and visualize what he wants happen. The words themselves don't matter so long as they state specifically what they want happen and contain proper intent (if it meets this requirement it doesn't matter how ridiculous it is - it will be equally valid). As a final note, spells need to be possible, ergo you cannot turn into a vampire.
I definitely agree with what has already been said, but i wanted to add something.
it is not the incantations, spell structure, language, etc. In my opinion It is the fierce will, desire, which can only be cultiavted by someone who knows the fine balance between what they need, and what they want.
I feel the methodology of magic is symbolism, metaphor, meaning in general, and it varies from person to person. My path is rich in symbolism, because that is what i am drawn to, for me it is the language by which i converse with the universe.
Nearly half or most of the spells in this site are doomed to fail, not because they are written wrong, but because reality does not allow for such things.