I started paying attention to SoM as soon as I heard that Monolith is studio making it. I loved FEAR, still play it every once in a while. Despite some of the talent leaving the studio and forming Blackpowder Games, (Betrayer) there is still some of the FEAR "flavor" recognizable in SoM.
I'll start with the visuals. Last game I played was The Vanishing of Ethan Carter so the bar was set very high and Shadows of Mordor doesn't quite get there. Image quality outdoors is ~on par with Crysis, however the polygon count is superior, in game characters are miles ahead but where Crysis struggled with indoors environments, Shadows of Mordor sidestepped interiors.
The sound and music were very good, worthy of the LOTR franchise. The name chanting of higher ranked enemies as they arrived to fights was a nice touch.
One does not simply walk into Mordor. First, one borrows combat style from Batman and elements from Far Cry 3 and Assassin's Creed. Hilarious that flawed parts of these elements weren't left out like leaving the cramped mission area, follow missions and stealth mission instant fails. While they could have made some icons look more different than Ubisoft's property, at least there weren't any eavesdrop missions so far.
I'm a huge FEAR fan, the game had outstanding AI that most titles today still fall behind. Enemies tried to surprise you, they knocked down tables and vending machines to take cover, they ran away if you killed the rest of their squad, etc, etc. While it's clear that part of the AI team left Monolith (AI path finding is bad and attack coordination is non existent, think riding a fully occupied elevator), they still have nice touches here and there. For example, if you battle a enemy with a rank that you fought against earlier, he may have bandages now and he'll "remember" the fight and mention a specific detail. The enemies tend to have conversations about you and I am under the impression the more you level up, the less do they dismiss the threat you pose to them. Another mechanic that is controversial is the enemy leveling system if you fail to beat them, they level up, becoming even harder to kill. Later in the game you gain ability to issue death threats to them. If you do, they hire bodyguards.
While all these things alone do not do much, combined they help immersion and makes fighting enemes more personal and the victories more enjoyable. The conflict within enemies ranks was also a nice touch and while the enemy ranking system was OK, it's refreshing to see a new approach.
Overall, I like Shadow of Mordor. To the point that I am savoring the game, playing in shorter session rather than "sprint" through it big strides just to finish it. Executing grounded enemies was a little clumsy and as other mentioned, I also had the problems of mistaking Berserkers for "regular" enemies. I've leaned against walls instead of simply walking by them but the game more than makes up for it's flaws.