First things first. If you're a LOTR purist, this game will probably annoy you, as it basically concerns a group of people going along and clearing up the mess that the Fellowship left behind.
Right. Here we go. The combat system and way you learn new skills in this game are exactly the same as Final Fantasy X-2, down to the way the menus are presented. You have a skill tree, and you learn new abilities by repeated use of skills lower down this tree. Even a lot of the abilities are the same as FF, with the item theft, armour shattering and the like. (Admittedly these aren't amazingly new ideas in an RPG, but nonetheless if I were squaresoft I'd be phoning my lawyers.)
The Third Age is a lot more linear than FF, though. You basically stroll along, kill some monsters, enter a different area, kill some monsters, levelling up very very rapidly all the while. Despite myself, I find this hugely enjoyable.
The Evil Mode is an interesting addition - once you complete a chapter, you get to play it through as the forces of Sauron, releasing new items for the main game. Don't be fooled by the ads asking if you will fight for the fellowship or destroy it. The Evil Mode is very much a bonus game, and not a Fable- or KOTOR-style different way of playing the main game.
Final criticism: I don't see myself playing this again once I complete it. There would be very little that could be done differently and playing exactly the same game twice doesn't really appeal.
So yes, this game has its flaws. Despite these, it is ridiculously enjoyable to play and the plot is fairly well done, in the way it is cut in with scenes from the LOTR films. Logically I know this game isn't that great but it's been a while since I've had so much straight-forward fun playing a game. (Incidentally, to my mind, this RPG style of game suits LOTR a lot more than the hack-and-slash combat that defined previous LOTR games.)