Executes sound dangerous for PvP reasons. I had a warlock in a BG hit me for over 200k. Now of course I have to be way below that health to actually take that much damage, but I suppose shields and other temporary damage reductions are easily vaporized unless the "effective health" they would normally give you would prevent from the execute ability from being used.
Also: I like it that any excess mastery from my previous target can be unloaded on the next target. In a fight with lots of smaller adds that are killed in small sets (or one by one), it benefits us that some of what would have been frostburn overkill is actually useable as icicle damage on the next target.