DoT Refreshing
Another change made to DoTs in 4.0 is how DoTs behave when being reapplied or refreshed. Basically if a DoT is refreshed between the 2nd last and last tick, the DoTs duration will be extended to include the last tick. For example:
Unstable Affliction (uneffected by haste) has a 15 second duration, dealing damage every 3 seconds (5 ticks). If you refresh Unstable Affliction at 13 seconds (2 seconds till the last tick) the new Unstable Affliction applied to the target will have a duration of 20 seconds and will tick 6 times.
What this means is that you can 'clip' a DoT without losing the last tick of that DoT (and the damage that last tick provides), making it much easier to have a 100% uptime on all DoTs. One thing to note is that if a DoT is refreshed using this method then the last tick inherits the stats of the new DoT (See 'DoTs, Buffs and Debuffs').