Customer expectations

I know this will seem very obvious, but my experience has taught me that the "obvious" often gets forgotten int he excitement of a new project... 

One of the most important things in getting user buy-in is to meet their expectations. The "nice to haves" can definitely add a lot of "wow" factor but no amount of wow will compensate for those basic requirements being unmet. Also, too much extra "wow" can often lead to confusion.

An important extension of this when dealing with Sharepoint is that when demo'ing to your customer always demo the different user groups - just telling them that different users will see different webparts / content / etc as required is not sufficient - you need to show them and the "sign on as different user" makes this quick, easy and painless so use it!