Set bonuses are the perfect time for Blizzard to experiment with fun class effects. They're in use by a limited set of players, they can be tuned specifically for a particular tier, and the bonuses are ephemeral. Class changes are much harder to implement quietly and they're even more difficult to roll back if undesirable. Tier set bonuses are a cultured petri dish for new ideas to grow, or to be sterilized. Our new tier 16 set bonuses are just such an experiment.
The two-piece bonus saves us energy on our combo point generators, and has some very interesting interactions with each talent spec's mechanics. The four-piece bonus also changes based on our spec. Killing Spree's damage ramps up significantly, which will pair nicely with its new Blade Flurry interaction. Vendetta's mastery-stacking bonus will add some teeth to assassination's burn cycle. The Backstab/Ambush combo pack, however, is easily the star of the show, and the start of an important conversation.