I dunno, I always found him to be excellent. While its true that it isn't that disimilar to LOTR, that's partially what Mark wanted, and personally, that raises two points that I see.
1. LOTR set a lot of standards. Most stereotypes and standards for fantasy, while not necessarily created by LOTR, were largly enforced by it. Overally, LOTR covered a lot of the fantasy genre. It'd be hard to write a book that's not at least SOMEWHAT similar to it without it becoming totally alien (if you wrote a book where dwarfs are big, tall, lean men with pointy ears, it'd probably be criticized for that because people are used to dwarfs being short, stubby men, thanks to LOTR.
2. LOTR was awesome. Doesn't that make Sword of Shannarah awesome too?
It's worth noting that Sword of Shannarah was the first book he wrote, and the other books in its same series are excellent too. Personally, Elf Stones of Shannarah (I do believe that was the direct sequel to Sword, though taking place in the future, with new characters, abet, related) was the best book, and one of the best fantasy books of all time too. Wishsong was good too. The other Shannarah series were less related, but still quite good. Allanon (sp?) is really the only character who reappears in all the books because he
appears even as a shade (ghost) after he is killed in one specific book.
All in all, whether or not you agree that it copied some elements, it is still a good book.