Even in this short period of time, I've hit several things that prevented or delayed a package from becoming stable. I think it's a good idea to warn people before rather than after, so if you want your package to become stable, please watch out for any of these:
- unstable dependencies; just file bugs for these first
 - failing tests; you should be running with FEATURES="test" all the time to catch these
 - upstream considering the version experimental; why should we consider it stable then? There are exceptions to this rule however if older versions are unmaintained.
 - open bugs for the package
 - repoman warnings for the package, or QA concerns on manual ebuild inspection (like not calling die on emake failure)
 - requiring the user to do manual configuration that could be instead performed automatically that if not done prevents the package from working completely
 
One more note: does every minor version bump need to get stabilized?