While yum brought a number of improvements, it also brought a hidden problem: the dbcache.
Yum archives downloaded packages/updates and tracks them with sqlite files in /var/yum/cache. Downloaded packages are not automatically deleted, and neither are the cache files, which can chew up a lot of disk space over time. The remedy is to periodically delete the cache files with:
yum clean dbcache
To remove downloaded packages, use:
yum clean packages
To remove cache, packages, and metadata (must be downloaded again the next time yum is run), use:
yum clean all