One of the most useful and innovative changes to the army rules in the new Orcs and Goblins book, in my opinion, has to be the new Choppa rule. Instead of the unique handweapon bonus of the old book, now all Orcs receive +1 Strength in the first round of combat as they build up their fighting spirit and let it all out in one go before they tire somewhat. The choppa bonus is of course cummulative with other bonuses, such as specific weapons, and does work on magic weapons as well.
Initially, many people immediately thought about Black Orcs now being able to dish out S7 hits with their great weapons, which certainly is impressive but isn't by far the most useful change (What, S6 wasn't enough for you?). On the other hand, it does turn the elite orcs into the designated Monster hunters. Got a Treekin or Hydra problem? Send in the Black Orcs with the Banner of Eternal Flame and watch them roll to wound on 2+.
In my opinion, the ones that benefit the most from the Choppa rule are mounted Orcs, specifically boyz on Boars or in Boar Chariots. With spears already granting +1 to strength on the charge from mounted troops, it effectively means that orcs carrying spears now hit like lances. This is an important change, as it turns these units from more or less disappointing pricey support units that had to depend on their mount to kill anything, into hard hitting shock troops, where the attacks from both the mount and rider can turn a battle in your favour. As such, I recommend always bringing spears on your mounted troops if you're able, and always buy additional orc attacks for your chariots if possible.
Finally, the Choppa rule makes equipping your Orc Bosses quite interesting and sorts out the disappointment you'd feel for the old Orc Heroes, who used to come in with a combat proweness similar to an Empire captain. It also makes our Warbosses capable of easily hitting that magic S6 which is vital if you want to take on tough troops such as Saurus or Chaos Warriors, as the Warboss will still wound them on 2+. With all that in mind, you usually will want to equip your Boss to have more attacks to really make his first round of combat a meaningful one. Depending on how many defensive items you're after, either the Sword of Slaughter (+3 attacks) or the Sword of Strife (+2) make for solid choppas. Or you can take the cheap road and merely get a Sword of Striking (+1 to hit) to really make the attacks you have hit true, while saving points for other things.
All in all, the new Choppa rule creates a lot of new opportunities for Orc units to shine in specific roles, which I think is a good thing. In my opinion, this is the most important change and upgrade given to us in the new book, even more so than the aforementioned changes to animosity.