Current version of Android and zero bloatware. Usually that alone gives a performance boost plus some custom ROMs overclock processor for additional performance.
The only reason I un-rooted and went back to stock was to get the update, then I'm going right back to root. The benefits are to many to list. Between the limitless customizations, overclocking (as previously mentioned), ability to remove/add apps at will, removal of all bloatware, etc. Be careful and pay attention when rooting and flashing ROM's and always do a backup, but once you do it will open your world to limitless possibilities and bring your phone to its true potential.