This game is fucking A. Usually the best you can hope for in an cash-in-on-IP-game is that they provide a solid game based on known mechanics, basically copy a good game and make it work through the IP alone.
I've only played an hour or so, but Shadow of Mordor actually has a few features I haven't seen before in any game yet. Get killed by a random uruk, it actually promotes the uruk to captain and gives him a name, and he'll come back at you twice as strong later.
Also in any other game if you get surrounded by ten guards, you usually just die. Here, they'll form a circle and come at you only 1-3 at a time while the rest watches. Fighting through a large pack might take a minute or two, but never before could a make actually make you feel like one of these story super heroes that can take on infinite Uruks like Aragorn. Every time that actually happens and you win, it's fucking awesome.
The other thing I really liked is how Stealth here actually makes sense. Kill a guard in thief and have it spotted? The other guards reaction makes no sense, eventually they even go back to idle.. in Mordor, one uruk said after they looked around for a while "Uruks die all the time, this was probably just a brawl". It's not that much different from Thief stealth, but in this setting it's not face palm 90s stupid.