I am afraid that list might be too long... and it would require input from large number of developers. It all depends on code you have and what kind of workflow you have at any given moment and for which platforms you develop. What is good version for some people, might be utmost nightmare for others. And besides stability, there might be other reasons and bugs in the core frameworks which makes some versions unsuitable for some developers. Workarounds were usually move to another version and hope for the best. Without actually trying on your own code and working with some version, one can never know.
Since Embarcadero opened up beta testing for larger audience, it would be good that anyone who can participate. The more the merrier. It is also important to start testing early, because some bugs if found too late in the cycle may end up unfixed. Of course, all that also does not guarantee final quality and release success, but it can be significant factor. Literally one person can stumble upon critical bug that would end up in RTM, and all other testers might miss it for one reason or another. It is huge codebase, and we cannot test and use all features thoroughly.