Habits.
Although there are plenty of reasons to love life in China, there are also a few Chinese habits that might take a little more getting used to. Probably the first thing that hits most foreigners face (luckily only figuratively speaking) upon arrival in China is the impression that everyone seems to be spitting.
The habit of spitting is certainly not limited to mainland China, although it does seem to be much more prevalent here than in many other societies. Despite the government's efforts to outlaw public spitting (particularly in Beijing leading up to the Olympics) and the fact that an increasing number of citizens - especially women - consider it to be inappropriate, it is still a common sight (and sound). Having to dodge the occasional "snot rocket" (also known as a "farmer's blow"), however, may test the limits of even the toughest "laowai"...
...which brings us to our next topic. After only a few days in China, Westerners will undoubtedly already have heard the word "laowai" (老外) repeatedly uttered in passing. This casual term for "foreigner" (typically used to describe caucasians) is considered by some foreigners to have a more negative connotation than the formal term "waiguoren" (外国人) - particularly when combined with hushed comments and furtive glances (or blatant staring).
It is important to keep in mind that, while the foreign population is estimated between 150,000 and 300,000, the native Chinese population has reached a whopping 1.4 billion. This makes the percentage of foreigners living in China a mere 0.2% or less (and a great portion of these are not from Western countries), so seeing a "laowai" may be something quite unusual for many Chinese.
Smoking is extremely widespread in China. The estimated number of smokers is 320 million (more than the entire population of the U.S.). In fact, about 50% of the male population smokes, but this habit is much less common among women. Many women also do not consume alcohol, although this is changing, particularly in larger cities. Other things foreigners sometimes complain about are aggressive market sellers, people cutting in line, chaotic traffic, and pushing and shoving when using public transportation.
Chinese habits are, of course, not all quite as controversial or obnoxious as the ones already mentioned. For example, it is not uncommon to meet middle-aged or elderly persons wearing their pajamas out in a park, hutong or supermarket. These Chinese may - according to the laws of Vogue or Glamour - be committing some major fashion faux pas, but this habit can't be anything but harmless.
Other, perhaps more internationally known, habitual customs are that of taking shoes off before entering someone's home (and changing into house slippers provided by the host), using chopsticks to eat, being exceptionally good hosts, showing great respect for the elderly, and habitually serving some pretty darned delicious cuisine.
Some advice? Get over the things that annoy you and start to enjoy all the other great things about China... or you'll be seriously missing out on everything else it has to offer.