What is the most important idea in Jane Eyre?
For me, the most important aspect about the novel is the conflict between passion and reason, or desire and logic. We see this conflict in Jane herself as she starts off as a girl unable to control herself in Gateshead when her cousin attacks her, and she "flies" at him "like a wild cat". In Lowood, thanks to the influence of characters such as Helen Burns and Maria Temple, she learns to restrain and control herself, being able to bear the unjust punishment of Brocklehurst without going into a rage. This enables her to defy her passionate side and resist becoming Rochester's mistress when she discovers the existence of Bertha Mason. Lastly, of course, she is able to strike a happy balance between the both sides - she is able to marry for love but this is balanced with duty towards her legal husband who is a dependent upon her. Key to this conflict is the way that other characters represent either side of the conflict: for example, many critics argue that Helen Burns (representing self-sacrificing logic and reason) and Bertha Mason (representing the excesses of passion) are alter-egos of Jane's personality, representing the worst parts of both these characteristics, and Jane is only able to live happy once these two parts of her self have been "killed off".
I think that the lasting legacy of Jane Eyre is that it is one of the first feminist novels. The character of Jane (admittedly plain) does not follow the traditional literary conception of a woman having to be beautiful and conform to stereotypical identities in order to live a contented life. Throughout the novel we see instances of Jane's strength and perseverance, motivated by her own voice, her own mind, and her own sense of self. It is this idea, this conception of being a woman that is so profound in the novel. I don't think you are going to find numerous examples of this in literature of the time. In fact, one can say that the idea of Jane establishing her voice and appropriating the world in accordance to this idea is the driving force behind the book. Bronte writes a character that is of the equal level and capacity of a man. The idea that women can pursue lives of their own accord, of their own choice, and of their own passions and can be happy in doing so. Jane's happiness and spirit of contentment lies in the fulfillment of her own autonomy and freedom. This is a radical idea in literature of the time, and to some extent, is still quite revolutionary in its articulation.
In addition to discussing Jane's independence as a woman, you could also discuss the idea of gaining independence from one's culture. As you most likely know, the Victorian era was rife with strict social codes. Jane's whole manner of thinking about herself and her worth stems from the era in which she lives. Likewise, Rochester's struggle with what to do with his insane wife is intensified because of the Victorians' view of marriage, insanity, and social classes. While Jane strives to discover who she is and what she is capable of accomplishing, Rochester must also decide how to handle his secret, pressures to marry, etc. All issues that would not be so pressing if Rochester lived in a more modern time period. You can certainly find other examples of characters searching for their own independence in spite of the rigid Victorian system.
What I find most admirable about Jane is similar to what accessteacher says. I find her adherence to principle in the face of great pressure to be inspiring and uplifting. When the man she loves, Edward Rochester, asks her to go away with him to a place where no one could possibly know who they are or the circumstances from which they come, she would love nothing more than to say yes. He's right; no one would know. But she would know, and she must be true to herself. Morals, she says, are not for the easy times but for those times when you can barely think straight and want to follow your heart rather than your head. If they are no good in those times, what good are they at all. I agree.
What about the ideas of equality and love? Jane struggles to understand her place in society and will not be with Rochester until they are truly "equal" (and his wife is dead.) Why does Jane only consider herself Rochester's equal once she has inherited money and he has been disfigured?
Is the "love" between Jane and Rochester really that at all, especially in the beginning? If Rochester truly loves Jane, why is he always finding little ways to change her? Why, for example, does he push her to buy fancy clothes when she is more comfortable in her drab ones?