There are many things that constitute a Light Jedi and I shall begin with attempting to find the most crucial ones.

The first two that reveal themselves are: a strong connection to the Force and service to harmony and balance (through serving a greater purpose and service to others). Of these two the latter would seem to be more important, for a Light Jedi uses his connection to the Force to bring balance to the Force and help others, making his connection a means, although it could (and should, I shall expand on this in the next section) be considered an end all on it's own as well. And also, if a Jedi were to lose his connection to the Force (which I think is actually impossible, but let us allow it for the sake of argument) but would still strive for balance and harmony in the world, he would still be a Light Jedi, at least in my opinion.

The reason why I also said that having a strong connection to the Force should be an end in itself for a Light Jedi, not only a means to achieve balance, is that it paradoxically seems that when someone has a very strong connection to the Force and when they start listening to the will of the Force, things tend to naturally start moving towards balance and harmony. This might be what a Jedi Master does, he is less about doing and achieving harmony by use of the Force and more about being in harmony and creating balance through succumbing to the Force, truly understanding that everything is connected and letting the Force work through him.

I think those are the main aspects that define a Jedi. But not the only aspects. A Jedi must also have a set of principles, for balance, peace and harmony in the World and eventually Oneness with the Force is not achieved easily and definitely not quickly and it is important to keep on the path and not to stray. Also it is important to remember that ends do not always justify the means, but might instead corrupt them. (I think this has been the reason why many well-intentioned Jedi have fallen to the dark side, but let that be a matter for later). In any case, a Jedi must have principles to guide him, and these principles also help in defining a Light Jedi.

• A Jedi is always looking for peace, balance and harmony.
• A Jedi lives in accordance to and in harmony with (the will of) the Force.
• A Jedi seeks to understand the laws of action and reaction, especially in his own actions, in order to make sure his actions truly bring about the positive change he is striving for.
• A Jedi does not wish to greatly affect the world around him, but merely to do what is necessary for positive change (a shift towards harmony and balance) to occur.
• A Jedi strives for development and growth within himself and around himself. He understands that stagnation and blocked energy tends to bring about negative energy.
• A Jedi is a bringer of Light, he moves to see and show things as they are, in their purest and truest form. This includes telling the truth and seeking to see the world as it is, without clouded judgment. This is critically important, for how can one strive for peace when he does not recognise it?
• A Jedi understands that he must be mindful of himself and aware of everything in his mind to reach true clarity and balance.
• A Jedi does not only look for peace but he is also at peace with himself and the world around him.
• A Jedi is here to help others in their respective journeys.
• A Jedi thinks before he reacts to a situation, (though trained reactions are also very useful in some situations) but he is also in touch with his intuition, which he also actively uses in his thought-processes, for it is an amazingly powerful tool and he also knows when to simply trust his instincts. (Eventually, a Jedi might reach a state when he is so attuned to the Force that his first reaction is always in harmony with the Force.)
• A Jedi cherishes all forms of life, for they are all wonders of the Living Force
• A Jedi realises that everything is a connected whole and does not think of himself as being more or less than anyone (or anything) else.
• A Jedi is in awe of the wonders of the Force and is therefore humble.
• A Jedi keeps his mind in the here and now, for it is the only reality. He does not let the burden of the past nor the fear of the future take control of his life. They are illusions.
• A Jedi seeks to control his body and mind, for they are the tools given to him to reach his goals here on this Earth.

I think that is all for now. As said previously, this definition will be ever changing, but I think I got down a good bit of what I think on this matter at the current moment.

May the Force be with You.