The biggest issue in WiFi, would be placement.
Before setting up anything, you should do a site survey. Walk around your home or office and look for things that can cause interference. Map things out, and take notes. Take a laptop with netstumbler installed and look for other wireless networks in range and see what channels they are on. Some things that can cause problems are:
*Cordless Phones/Microwave Ovens: These devices operate at 2.4 Ghz, which is the same frequency Standard 802.11G/N routers use. Since they both operate at the same frequency, they will interfere with each other's signals.
*Concrete Walls: Concrete causes a problem, not because of the thickness or the re-bar in it, but because of the water in the mixture. The magic thing about concrete is that it never stops curing, and the water in the wall can cause signal loss.
*Pointing your antenna: Some people think that pointing the antenna is self explanatory, but it isn't. You can't point your antenna like you point your finger because the signal coming off the antenna radiates from the sides of the antenna, not the top (See the figure below, I am not an artist!)
*Wireless Channels: By default, almost all routers come pre-configured to use either channel 6 or 11 (if I'm not mistaken). Find out what channel the surrounding networks are using and select a different channel. As mentioned above, Netstumbler for Windows works well for this as well as kismet for Linux and Kismac for MAC.
*Chicken Wire: That's right, I said chicken wire. I didn't come up with this one myself. I had a wireless class at college, and my professor told us about chicken wire. Chicken wire is used in some houses to either hold in insulation in the walls, or is used when putting up plaster walls. The holes in the chicken wire are not large enough to allow the sign wave of the wireless signal to penetrate the wall and thus creates a Faraday cage of sorts.
Issue number three, compatibility. Almost all wireless devices come with the WiFi logo which is supposed to indicate some sort of standard. It suggests that if you have a Belkin router, your Linksys wireless card should be compatible. Well, 9 times out of 10 that is true, but every once in a while there is that 1 time where it is not true. It is almost always best practice to stick to the same manufacturer. If you are a D-Link person, stick with D-Link, if you like Linksys, stick with linksys and so on.
Issue number four, Firmware. The manufacturers of your equipment frequently come out with firmware updates for their products. If you notice issues with your router or card, look on the manufacturers website for a possible firmware update. Lots of times, the update can correct the problems you are having.
Issue number five, loss versus gain. Every time you have to send a signal down a wire you have loss. If you have a long cable going from your modem, CSU/DSU etc going to your router there will be loss of signal. The longer the cable, the more loss. Likewise, the air creates signal loss as well. The farther you are from the antenna, the weaker the signal. On the flip side, whenever you have an antenna, you have gain. Every time the signal comes out of the antenna, or is received by an antenna there is a little bit of gain in the signal. So what does that mean to you? Shorten your cables if possible, and buy higher gain antennas for your routers if you need the signal to travel longer distances.
There are more problems I'm sure that I have left out. If you can think of some, please comment about it so others can learn more.