I think the reason its being so well received is because it just is a breath of fresh air in the gaming industry. Batman already did the combat, Assassins Creed already did the freedom of movement in an open world, but what Shadow of Mordor did was take two already proven concepts and put them together so well. The Nemesis system is what really puts this "good" game into "great".
Sure the rest of the game is just solid, nothing stellar, and the nemesis system alone isn't going to make a GOTY, but when all of it was done really well together we have something that we didn't know we wanted. Every game before Mordor has sold us good villains through story. Think about every game, the scripted events make the villains. They are something different than the rest of the enemy characters in the game. Mordor doesn't do this. Mordor gives you various NPCs, and says "these guys are nobody, until they do something great, then they become a somebody." Each enemy that you hate, you hate because of an interaction that you had with them, an unscripted interaction that has set in motion a chain of events that will change how you play the rest of the game.
Granted it only changes a little bit, seeing as no matter who does what you are still going to play the game the same way, but now it actually feels personal. No longer is every body chasing after the same named character, now they are chasing after someone different with different stories of how they came to be with different outcomes in the enemy line up. We have not only a proof of concept, but a damn fine game that has told us that a game can be emotionally charged with out being scripted.
How you play the game, even in Mordor with very little variety on how you play, will change how the game effects you.
For Example:
I killed an orc leader in a fire, he comes back and he has some scaring and some new metal face. When he sees me again he says "you thought you killed me in that fire did you?" and he kills me. Now he ranks up, gets more powerful, and I'm tracking the damn orc down because he's supposed to be dead, and he's not, AND he killed me. I'm pissed, and he is now my number one target. Fuck that orc in particular.
Thats my story. Everybody has a story in the game that is essentially the same, but yet it feels so much more personal for each player. This game on the merits of the gameplay alone wont win GOTY, but the nemesis system is so much more to the player than the gameplay that I believe it deserves the GOTY award.
Mordor has shown the audience that the new consoles are going to allow AI development beyond what we've seen, and with creative people behind them we can get somethings that we didn't know we really wanted. Mordor is less of an amazing game and more of an amazing concept with a bright future for video games. A GOTY to me isn't the best game, but rather a great experience.