In a prologue to the later editions of The Lord of the Rings Tolkien writes
The most critical reader of all, myself, now finds many defects, minor and major, but being fortunately under no obligation either to review the book or to write it again, he will pass over these in silence, except one that has been noted by others: the book is too short.
Apart from being too short, do we know what other defects Tolkien is referring to?