What I've learned so far:
- lots of people everywhere all the time <- Yep. Not sure about Beijing, but population of Shanghai can be considered as equivalent to Malaysia. And there are many other cities in China that are like that.
- good apartments are expensive <- Even small apartments are expensive!
- learning Mandarin is a good idea

<- It is not only a good idea, it's imperative.
- traffic is bad <- and you won't get used to the driving style for at least 5 months. It's horrible, much worse than kiasu malaysians, but of course, not as bad as India. You'll probably get a huge shock, sitting in a cab for the first time/trying to cross the street. By the way it's left hand drive.
- subways are good and cheap, crowded during rush hours but reasonable otherwise <- they are always on time (based on personal experience). and expect Malaysian ones to be a miniature toy, as over there, there are many routes (well connected), and it could be very confusing for the first timer, since majority of words/people to ask are in Chinese.
- cost of living is about the same as KL
- food quality can be bad <- it's just like KL, there are plenty good food, and plenty bad food. Just that China's land area is larger so you have more travelling to do.
- air pollution is bad, get a good quality air filter for the apartment and 3M facemasks <- good quality standalone water filter too in addition to the fixed rocket filter that might already be provided by your landlord.
- cycling is an option but could be dangerous depending on where you go <- cycling is dangerous anywhere on Chinese roads, except in and around your own housing estate/apartment area.
- there is double taxation if staying more than 183 days
- power supply is 220V/50Hz similar to Malaysia but the plugs are similar to Japan/Europe <- not a worry. the normal extension plugs (rack) are widely available and can support Malaysian plugheads. You can also buy a small portable converter head (approx RM30) that would allow you to bring your laptop anywhere.
- mistresses are expensive

<- mistresses are potentially detrimental to family life and career development, if they become cunning and demand ransom money from you to shut up. "Massage centres" would be a better and safer alternative, if you know what I mean, lol sorry if you're offended.