usability

If it is unusable, everything else your Web application does is redundant.  Massive improvements can be made by following straightforward examples and patterns of what works.

usability requirements
It is vital you consider all the users of your site.  A common pitfall is to assume to much about how they will interact with it.  Usability studies highlight the problems your site poses for different types of people.  The solutions suggested nearly always conform to patterns seen before - afterall user interface design is nothing new.  The results in usability tests taken after action are always impressive - such small changes, such a big impact.