I don't view it as a good idea. 500 characters for feedback? That is going to clog up the feedback profile pages, and I see the potential for feedback waffling. Instead of being forced to be concise, buyers will have enough room to include irrelevancies.

The 80 character limit was severe. Sometimes conveying sufficient information with that limit couldn't be done properly. However, 500 characters is far too many. If eBay were going to increase the number of characters, 100 characters for feedback would in my estimation be adequate.