The nature of order
Sauron was known and respected for his perfectionism. However, his desire for order and control ultimately led to his later transgressions. As such, this suggests that good traits can ironically lead to bad consequences.
Melkor and Sauron
Both Melkor and Sauron were created to be good and virtuous. However, they both ironically turn out very differently, threatening to destroy the harmony of Middle Earth.
Irony of success
The Silmarillion was written before The Lord of the Rings trilogy and was originally rejected for being too vague. This is ironic because it was then followed by the famous trilogy which was a huge success.
The Silmarillion
Although Tolkien's other works were a great commercial and critical success, The Silmarillion was not received as well. This is perhaps unexpected from a great author like Tolkien.
