Here is and email that I got from the HR lady in the HYD office, Shuba Menon:

Myths & Beliefs of Ganesh Chaturthi
Why does Lord Ganesha have a half-broken tusk?
When he came up with the idea to Mahabharatha, Sage Ved Vyasa looked around for someone who would write down the words even as he recited them. He approached Lord Ganesha and asked him if he would do so. Lord Ganesha readily agreed.
Ved Vyasa said "I will narrate the story and you must take it down as fast as I say it and should not interrupt or stop anywhere in between" and Lord Ganesha too said that "I will do so and if you halt or hesitate, I will stop writing and your epic will never be written". Ved Vyasa agreed with his final condition "So I agree but you must also agree to fully comprehend the meaning of the poems as you write and not just blindly write them as I say".
The elephant headed God gave his consent and they started writing the epic Mahabharata. To write this Lord Ganesha broke a bit off his tusk and used it as a pen.
As they continued with the writing, Ved Vyasa found that Ganesha was very fast and he didn't even have any time to breathe in air! His face slowly turned blue and he wondered how will he go on without pausing to draw his breath also. Then he came up with the solution and narrated a difficult stanza. Ganesha, who was writing at a furious pace, stopped for a second to get the meaning of the stanza and Ved Vyasa thankfully used the split second gap to gulp deep lungful of air. But Ganesha carried on immediately and they went on. And so, whenever Ved Vyasa needed a break, he would tell a difficult phrase and used the time Ganesha took to comprehend the meaning. Thus the original Mahabharata contains many difficult stanzas placed at intervals throughout the length of the epic.


Why people don't look at the Moon on that day?
There is very popular legend regarding this context. Lord Ganesha was very fond of sweet pudding or Ladoos. On one of His birthdays, He was going around house to house accepting the offerings of sweet puddings.
Having eaten a good number of these, He set out moving on his mouse at night. Suddenly the mouse stumbled as it had seen a snake and became frightened with the result of that Lord Ganesha fell down.
His stomach burst open and all the sweet puddings came out. But Lord Ganesha stuffed them back into his stomach and, caught hold of the snake and tied it around his belly.
Seeing all this, the moon in the sky had a hearty laugh. This unseemly behaviour of the moon annoyed Lord Ganesha immensely and so he pulled out one of his tusks and hurled it against the moon, and cursed that no one should look at the moon on the Ganesh Chaturthi day. If anyone does, he will surely earn bad luck.

Beliefs
In India, Lord Ganesha is worshipped first on all auspicious occasions, whether it is a marriage or a religious function. Lord Ganesha is the foremost god of the Hindu Pantheon. Any new project or venture that a Hindu family undertakes starts with his name, the housewife utters his name before even starting a small chore as he is the remover of all obstacles and is an extremely benevolent god, fulfilling the wishes of those who pray to him sincerely.
Lord Ganesha also has long been associated with commerce, and merchants still pay homage to him. In households, it's common for small offerings of money, flowers and food to be placed before one of the family's effigies of Lord Ganesha . These tokens please him and he therefore brings more beauty, money and food to the family.
It is also a firm belief of every religious Hindu that he / she must pray to Lord Ganesha before undertaking any job. Once, Lord Siva set off in his chariot to wage war against a demon. Soon after He set off, one of the wheels broke. Then He realised that He had forgotten to pray to Lord Ganesha. So, He went back home, prayed to His own son, and successfully defeated the demon!