Mordor and the Nemesis System
Monolith may have already released Shadow of Mordor, but that clearly hasn't stopped new trailers from coming out. The newest trailer, the "Forge your Nemesis Trailer," makes sure to point out what lots of people have been calling Shadow of Mordor's coolest mechanic, the nemesis mechanic.
If you didn't know already, Everybody's favorite Dark lord Sauron has no shortage of minions, and the most powerful of those minions are captains and warlords that command hordes of Uruks. Being the savage Uruks they are, they plan to hunt you until your dead and feed you to the caragors, but if you stop them and stop just short of murdering these captains they'll come back for you again, and again, and again, until you finally drop dead. The neat part about this is that when they come back, they come back prepared and beefier than ever. If you shot one half to death he might come back immune to ranged attacks, or if you snuck up on him and attacked him from behind he might become stronger against stealth attacks, they're basically leveling up alongside you.
Even cooler is the fact that you can "brand" your upgraded nemeses and have them do anything from kill another captain to give you more intel on what Sauron is having for breakfast that day. While Shadow of Mordor may be a little button mashy or QTE heavy for some, the Nemesis System must make up for some of that, I know it did for me.
Well what do you know, Monolith made another trailer about the Nemesis system as well.