Clarifications on the changes coming with global cooldowns, stop casting, and all that with patch 2.3:

In 2.3 you don't need /stopcasting, you can always try to cast before the end of your current cast. This gets the cast to the server sooner so it's more likely that your casts will go back to back.

A couple things to note:
* The global cooldown is client side, and you can never cast more often than the global cooldown (when it's triggered)
* This doesn't reduce your cast time. If you send your cast too early, it'll still fail because another spell is in progress.
* Careful timing is always better than spamming. Prior to 2.3, you were always penalized by your latency when casting. Now you're only penalized if you cast too early. :) People who used to use /stopcasting are familiar with this, but now the penalty is simply a round trip from the server instead of canceling your previous spell.