In my opinion this is an often over looked and under estimated part of most home build projects.  Most people will spend quite a bit of time in deciding which motherboard or processor to use but simply pick a case based on price.  Remember a case can be used for years, it will probably survive multiple processors. motherboards etc.  Since most all motherboards currently used are of the ATX form factor your choices are somewhat simplified to desktop, mini-tower, full-tower and all those in between.  Once you start looking at cases you'll see what I mean the choices are in some ways overwhelming.  One site that illustrates the wide variety available is colorcases.  Your best bet is to figure out how much room you need for drives and how much space you have available for the computer, in order to start narrowing down the field.  Also keep in mind how easy it will be to get into the case if you are going to be tinkering with it on a regular basis.  Look for cases with rolled edges or cut-free construction.  Nothing is more annoying than working on the computer and getting all cut up from sharp edges.  Also make certain that the case meets your processor and motherboard manufacturers specifications.  As stated in other places on this web, cooling is becoming an ever more critical part of proper system operation.  Does the case have the proper airflow to meet these thermal requirements.  Is it well built and thought out.  These are all things you most consider.  

As with all other aspects of home building there are many resources available on the web.  Most of the tech sites have case reviews.  Start reading them to find out about what's available and what are good things to look out for.  You'll also find all kinds of sites dedicated to case mods (modifications).  From things as simple as painting it a different color, to as complex as airbrushing designs, installing blowholes and windows.  Most cases are primarily sheet steel, some are mostly plastic, and now aluminum is gaining in popularity.  Some cases are available that are made from clear plastic, although with no metal to shield it, it will cause quite a bit of RF interference. 

Also do not neglect the power supply.  Make certain the case either comes with a good quality PS of sufficient wattage or get the case without a power supply so you can add your own.  I have seen many system exhibit problems that were ultimately due to a bad power supply.  Random GPF's, Blue Screens, reboots, data corruption, etc.  All due to bad or cheap power supply.  Use common sense, it is unlikely that a $40 case will have a good quality power supply in it.