I would have thought there are two main elements that you can look at to show the feminism that is explicit in this excellent tale: the "point" of the tale and the character of the wife herself.
We see at the end of the tale that what ensures the poor knight gets a beautiful and virtuous wife is when he correctly gives mastery or dominance to his wife, giving her the power to choose what is best for them. Note what the knight says to his wife in the story:
"My lady and my love, my dear wife too,
I place myself in your wise governance;
Choose foryourselv whichever's the most pleasant,
Most honourable to you, and me also."
The message is clear: happiness is given to men who let their wives rule them. It is hard to overestimate the anti-patriarchal message that this represents, as it challenges so much of the culture of the time.
You might like to think as well of how the character of the Wife of Bath in her very person represents a feminist statement. She quite happily declares that she has married many times, each time to her advantage (mostly), and that she is on the look out now for the next one. Note the way her character is revealed at the end of her tale:
And may Christ Jesus send us husbands who
Are meek and young, and spirited in bed;
And send us grace to outlive those we wed;
And I pray Jesus to cut short the lives
Of those who won't be governed by their wives...
Again we see a strikingly bold and open rejection of the role that society of the time had created for women, especially with reference to the fact that women prefer men who can sexually please them. Such a vibrant character completely shocks those who expect women to be mild, demure and subservient.