I don't think this has been discussed before but when requesting something, my books tell me that "xiǎng yào" is more refined and polite way of asking for something than just "yào".
Wǒ yào yī hú lǜ chá.
我要一壶绿茶。
I'll have a pot of green tea.
Actually, this would more accurately translate to "I want a pot of green tea". The more polite way to order is
Wǒ xiǎng yào yī hú lǜchá
我想要一壶绿茶
I would like a pot of green tea
Any comments?
xiǎng yào versus yào

Robert-C7
August 21, 2014

Lin-Ping
September 2, 2014
Hi Robert,
Yes, you are correct, 想要 is a more polite way of ordering. However, we don't often use it. If we wanted to be polite we would often say something like,
来一壶绿茶吧。
Lái yī hú lǜchá ba.
Can I have a pot of green tea.
Having said this there are many different ways of ordering in China. Many of them depend on the region and dialect. Also what is worth remembering is that politeness in China when ordering in a restaurant (compared to the West) is almost not existent. Yelling at a waiter from across the room would be considered horrific behaviour in the West but in China it is perfectly normal and not seen as rude.
I have digressed a little but I hope this helps a little bit.
- Lin Ping

Robert-C7
September 2, 2014
That is interesting about politeness regarding servers in China. Here in Southern California, service is always rushed even if you ask them to slow down. If I want time to eat my hot sour soup before having all the entrees served or worse, having the entrees served before the soup, I need to order the soup and let them serve it and hold off ordering the rest until we have had some time to eat the soup. I usually go to the restaurants where Chinese people go since my wife is first generation Chinese.
The few restaurants that do not rush but follow American protocols are usually not authentic Chinese restaurants and my wife usually complains. This includes PF Chang.