Mormonism: Why is the temple so important?

Anyone familiar with Mormonism probably has had some sort of exposure to the concept of temples. But many people are somewhat mystified by these buildings. What is their purpose? What goes on in there? And why does the Church of Jesus Christ of Latter-day Saints stress so much importance on the temple attendance of its members?
Everything that goes on in Mormon temples is centered around the family and their relationship with Jesus Christ. All things done inside are to ensure the salvation of all and to bind families together for eternity. Simply stated, members of the Church go to the temple to perform essential ordinances and to make covenants with the Lord. It is a place of peace, a House of the Lord. Members go there for clarity, to reflect, to be nearer to God, to pray.
Another key purpose of LDS Temples is the redemption of the dead. Ancestors who died without baptism or the other essential ordinances of the Temple can do so by proxy. A living member of the Church, worthy to enter the temple, may perform these ordinances on behalf of the deceased by being baptized, confirmed, etc. in their place. Some outside the Church take issue with this, as it seems to them that the Church is forcibly converting people after death. But this is not the case. Members can only submit the names of direct ancestors, and if they wish to submit someone else's ancestor, they need express permission from a living descendant. According to the teachings of the LDS Church, there is an element of choice inherent in these proxy ordinances. Those on the other side have the choice as to whether they accept the ordinances, just as a person would have a choice here on earth. Members serving in the temple are merely opening a door for their ancestors; whether they walk through it or not is their choice.
One question often leveled to the Church is "Why is there so much secrecy surrounding the temple?" Most of what happens inside remains a mystery to non-Mormons, and even to those Mormons who have not been through the temple yet. Those who do know, usually don't talk about it outside the temple. If it's such a great place, why can't everybody know the intricate details?
There are a few reasons for this, the first being the sacredness of the temple and the mysteries learned therein. The temple is a sacred place, and only members of the Church who are worthy--meaning they obey the teachings of the Church to the best of their ability--may enter inside. These individuals may enter because they have disciplined themselves, and have proven themselves in the eyes of God that they can respect and cherish the blessings and teachings they receive inside the temple. Those who go through the temple have usually learned enough that what happens in the temple makes sense on some level, and has context. This is another reason that the inner-workings of Mormon temples are not trumpeted from the rooftops. They lack context. Without an understanding of the background of these ordinances, and their symbolic and spiritual implications, it's easy to discount them, to make light of them, and disrespect their holiness. The perceived "secrecy" is to protect the holy covenants of the temple from ridicule, and also to protect those who might mock them from judgement, as they do not have the full picture.
This is not to say that one trip to the temple can give you all the knowledge you need or instantly make you perfect. Members often describe themselves as continually learning new things, gaining new insights on the Gospel of Jesus Christ, and gaining personal revelation for their lives. Even prophets, the men who lead, and receive revelation for the whole Church, have said that they are still learning on each visit to one of these sacred houses of the Lord.

On other sacred ordinances of the LDS Church, read this article about the importance of the sacrament.