In 2.3, Mystical Skyfire Diamond is getting a cooldown but an increased proc chance.  The current consensus (without testing) is that the cooldown will probably be 45 seconds.
Right now, MSD has a proc rate of 5% with no cooldown.  This averages out to approximately 1 proc per minute for Fireball.  In order to continue to be 1 proc per minute with the new cooldown, the gem has to proc within 15 seconds - 5 Fireball casts.  So the proc rate would need to be 20%.
Allegedly someone has determined that the new proc rate is 15% which would make it effectively half a percent less than it currently is.
If the proc is less than 20% and the cooldown is 45 seconds, MSD will likely come out at a loss to the new caster metagem (the one replacing RED).  (This judgement is based on mucking with Vontre's mage spreadsheet and many not be particularly accurate.)
I'm just annoyed because I finally got Legwraps of the Aldor and commissioned a MSD to be cut.