Ease of use
When designing (or selecting) a legal extranet, be sure it is easy to use. If it is not easy to use, it will not be used.
How is that statement "make it easy to use" defined?
Make all of the screens consistent in their presentation. Be sure all the system navigation menus are deployed and displayed in a consistent manner. Make the text readable -- no 6 point fonts or crazy colors. Display only a reasonable amount of information on each screen - too much information can be overwhelming for people to process -- especially those who might be accessing your system on an iPad or smart phone.
From a technology standpoint, use technologies that will work on everybody's internet browser (there is nothing system clients hate more than to have to download internet plug-ins) just to get something to work. Consider cool toolsets like jQuery to provide advanced functionality to users on a thin client.
And, select technologies that will allow your clients to load large volumes of information (no flat file databases, no other obvious size constraints) so large litigation support projects can be handled by the legal extranet.