This expansion is worth every penny.
Farmers of the Moor adds several new concepts to the original game: heating and farm clearance. You now have to heat your home with peat or wood and you can clear your farm spaces of moorland or forests to create peat or wood. As you might expect there are new minor improvement cards to use these new resources. What are not included are any new occupation cards. This is important because if playing with elements from this expansion, you are advised not to use the occupations. This is a reduction is complexity and component interaction to counter the introduction of action cards that do not cost an farmer to use. The action cards are a really interesting addition to the game: they can be taken by any player instead of placing a farmer on board space and generate one of three abilities often relating to moors or forests on a players farm or allowing an action at an increased cost. These cards add a lot to the game as they break the farmer placement flow and players have to think about taking a prime space or getting another action opportunity later and taking an action card.
The Family Game of Agricola (without minor improvement cards or occupation roles) expands very well into Farmers of the Moor as it allows slow complexity progression without piling on a lot of components all at once. Sure, if you are used to playing a full complexity game of Agricola and then add in all the expansion options at once then it's going to be complicated, but it needn't be that way. There are a lot of customisable options available.
Finally a word on horses. The expansion comes with wooden horse markers which is fantastic, further incentive to replace the cubes in the original game with similar items. horses give you a point per horse and they have to be stabled as other animals. However, they can come into the game earlier through the action cards and a player can collect a lot of them by the end of the game, should they want to. This is other way to specialise rather than generalise in how you play Agricola and worth noting as a criticism levelled at the game is that is railroads players into generalisation- a view I disagree with. I particularly like playing with the expansion as it removes situations where players queue for the occupation spaces, which can be frustrating on occassion. I am not of the opinion that Farmers of the Moor makes Agricola more complicated, it depends upon your starting point and how you enjoyed the original game, but is certainly adds new experiences to playing the game.