Apparently, there were a few reasons. He wasn't keen on the overt religious themes, but what really grated with him was the inconsistent "borrowing" from different cultures and canon to create Narnia. (Such as the use of the minotaur, the snow queen, griffins, elves, etc.) Tolkien spent a long while building up this world for Lord of the Rings that was carefully composed of its own language, mythology and such, and he felt that Lewis didn't put in the same thought about it.

There's a really good article about it here, and one in Italian, too: ;)

http://books.guardian.co.uk/departments/childrenandteens/story/0,,1657756,00.html

http://www.fantasymagazine.it/rubriche/125