@nickon thanks for the suggestions. Most of these are already on my to-do list, with various priority.
I will probably not add “write a review” under “add to cart” button when review already exists - to me it's just unnecessary clutter.
From my point of view, if user wants to submit review, he will quickly find Create review button down on the page. More importantly, regular visitor do not want to write a review at all, this will be done mostly by customers that have recently purchased this product. I think you should optimize your product page for potential buyers, not for potential reviewers.
For customers there will be the additional module that will send email and ask them to write a review for products they have purchased recently, and review link will take them to their customer account.
But if you need this functionality, you can easily modify views/templates/hook/product_extra.tpl template