There's been a lot of contention about THX certification recently, especially in the wake of the whole Lexicon/Oppo debacle. THX has decided to answer the questions by finally disclosing some of what goes into the certification process.
THX didn't give full disclosure, mind you, but they did allow for the publication of some of their specifications. Among the requirements for THX specification are low reflectance from the panel, brightness uniformity, and off axis color accuracy.
The guys at HD Guru, who got to see the testing process hands on, were given a complete copy of the specifications, but were only able to publish certain parts. THX says that this is for competitive and business reasons.
If you want to dig deeper into the specifics, you can find a more detailed list of the specifications directly from THX at the link below.
Source: HD Guru