Games based on Tolkien's works always have to involve some additions to the lore, otherwise you may as well just read the book. Sometimes these additions are good, sometimes not so good. For the most part Turbine has handled its additions in a way that make them very credible extensions to the story. Here's an example. At this point in the game, the hobbits have all reached Rivendell, and Frodo has accepted the quest of the One Ring. From the Council of Elrond we know that Gollum has escaped captivity in Mirkwood and seemingly did so with the aid of servants of the Enemy. What's this have to do with you? Well, while wandering the Trollshaws in search of adventure you hear rumors of creature that has been raiding fishing nets and even tried to steal a baby from a house. So, with the help of a local hunter, you set a trap for the creature and wait. What follows is an entertaining chase with a challenging encounter at the end of it. A fun quest line, with a nice solo quest at the end, and the reward of interacting with Gollum.
Not all of Turbine's additions are great from a lore standpoint though. Runekeepers? Really, what were you thinking? They're a good class from a purely game mechanics point of view, and adding a second healing class was definitely helpful, but seriously, this isn't very faithful to Tolkien. Gandalf started a fire to keep from freezing to death and said that he just painted a bullseye on himself for anyone to see, and Runekeepers are throwing lightning bolts around like there's no tomorrow. How hasn't the Enemy found all of them and either recruited them or put them down? Granted, several of the classes in the game have some rather overt displays of power, but RKs are nothing but overt displays of power.