The Oval shape is often emblematic of a yoni (Sanskrit for "divine passage") the symbol used to signify the female/great goddess. It's related to sex, certainly, but also (and particularly in this case) to birth. Thus, the lady dancing within that Oval (or Yoni), is not being limited. To the contrary, she is being born into a whole new universe of understanding, possiblities, experiences, etc.
That she is, herself, female, gives a recursive factor to the image (a female dancing within the symbol of the sacred feminine). She is giving birth to herself--it is her own efforts that have created this new world--which is why The World is often the card of scholar and those who are graduating or becoming masters. They have given birth to themselves (as it were), and opened up the universe for themselves via their hard work, study and effort. And being female (having that yoni) they can give birth to more--which is why it's the card of teachers; a scholar or master who teaches, gives birth to new scholars and masters.
Putting it another way, the oval does not signify a spherical enclosure, rather it signifies a doorway. I would only reading it as limiting (as the door closed) if it was reversed.