Think of Wireless LAN as a system

Wireless troubleshooting-tip: What makes up a wireless LAN? 3 three main parts:
  1. The infrastructure
  2. The clients
  3. The environment
The Infrastructure
This includes not only the access point(s) and the WLAN controller(s), but any of the routing & switching gear used for uplinks. Also included in the infrastructure would be any of the network infrastructure / services leveraged by the wireless LAN (examples include RADIUS, Network Time Protocol, Syslog, etc.)
The Clients
The IEEE 802.11 specification refers to these as "stations". Clients/stations could be computers such as laptops, table PCs, or netbooks. Other examples of wireless stations include dual-mode phones, handheld scanners, etc. When thinking about the clients, realize this includes not only the hardware, but the software (such as a client driver or management supplicant).
The Environment
This most commonly refers to the Radio Frequency environment. The RF environment can be evaluated with a spectrum analyzer, which can show you the signal level of your APs, the noise floor, and any interference in the area. Interference can come from other Wi-Fi devices, or non Wi-Fi devices (such as microwave ovens, cordless phones, and Bluetooth). I also think of the Physical environment, which is the type of construction materials used, the amount of vegitation in the area, the terrain/line-of-sight, etc.
Troubleshooting situations. In many of these cases, the infrastructure has been evaluated as a potential source for issues (for example, are the authentication/encryption settings correct), but the clients and the environment have been neglected.
In troubleshooting scenarios, it is imperative to take a thorough look at all three components. Not only should you take a look at the configuration file of the infrastructure, but ensure that wireless clients have the most up-to-date drivers. Look to see that clients power-save and roaming agressiveness settings are appropriate for the application. Double-check to ensure there is no interference from Wi-Fi or non Wi-Fi sources, etc. In short, evaluate your WLAN as a system.