What are Mormons allowed to discuss about the temple, when not inside the temple? What are we allowed to discuss or ask questions about outside of the temple?  What things exactly are too sacred to discuss outside the temple, and which things can we talk fairly openly about?
From lds.org:

What can I say to others about my experience in the temple?
You can talk about what the interior of the temple looks like, and you can freely share the feelings you have in the temple. However, temple covenants and ordinances, including the words used, are too sacred to be discussed in detail outside the temple. By avoiding discussion of these sacred things outside the temple, we protect them from mocking, ridicule, or disrespect. Do not be casual when talking about your experiences in the temple.

For much more on the topic of temples, what is sacred, and how to treat sacred topics with respect, see the special edition of the Ensign magazine, October 2010 issue. For pictures, try a PDF copy of the issue.
In particular, see The Holy Temple by President Boyd K. Packer. He says:

The ordinances and ceremonies of the temple are simple. They are beautiful. They are sacred. They are kept confidential lest they be given to those who are unprepared. Curiosity is not a preparation. Deep interest itself is not a preparation. Preparation for the ordinances includes preliminary steps: faith, repentance, baptism, confirmation, worthiness, a maturity and dignity worthy of one who comes invited as a guest into the house of the Lord.