Disability.
China has traditionally not been particularly accommodating for disabled persons. Luckily this is changing, and, especially because of the 2008 Paralympics, Beijing is instituting a number of changes to make the city more accessible to the disabled.
Most of the main tourist attractions are at least partially wheelchair accessible, with built-in ramps leading into the main areas and over some of the building or courtyard entrance thresholds (which are typically raised and can be as high as 30cm/1ft), although unfortunately steps can sometimes unexpectedly appear along a path marked with a wheelchair symbol.
Navigating Beijing's streets can also sometimes pose a problem for disabled persons. While some of the sidewalks around town are fairly wide and have flat surfaces, a great number of them, particularly in the older central areas of town, tend to be narrower. Along practically every sidewalk (as well as in the subway stations) there is a raised section, intended to assist the blind. Although this is helpful for the vision-impaired, it can be difficult for those in wheelchairs to maneuver along bumpy sections, particularly if there are trees periodically placed in the middle of the flatter section. Adding to these obstacles is the fact that practically everywhere one looks in Beijing, there is construction going on in some form or another. Often the easiest thing is just to avoid the sidewalks and share the street with the bicycles, cars and pedestrians. Obviously, Beijing still has a ways to go in becoming more disability-friendly.
However, newer facilities are generally being built with more accessibility - especially hospitals, hotels, railway stations, subway lines, supermarkets and shopping centers. Most large apartment buildings always have an elevator, and there are more and more being outfitted with wheelchair ramps.
Currently, all of the newer subway lines (5, 10 and the Olympic spur line) are accessible for the disabled. Unfortunately, many of the stations along the older lines 1 and 2 do not have elevators, and if they have escalators, then they may not lead all the way to the platforms. When trying to get around areas near lines 1 or 2, it is therefore best to take advantage of Beijing's multitude of taxis.