I typically recommend that people use Windows SharePoint Services unless
SharePoint Portal Server is mandated. What follows is my thumbnail list
of reasons that I think compel people to use SharePoint Portal Server
listed in the order that people seem to rank them:
-Enterprise Search (vs. Site search)
-My Site
-Audience Targeting
-Areas
-Can be hierarchically moved around whereas WSS Sites are fixed
-Can have Web Parts that reflect an Audience
-Can be set to expire at a given date (and other metadata)
-Can display the same content on multiple areas
-News/Listings (super links with metadata like expire date)
-Some unique Web Parts including rollup capability
-Links for You
-News for You
-Sites I've Created
-And more
-Categorized Site Directory
-Single Sign-On
-Stored Alerts
-Dynamic Top and Left nav base on Area hierarchy (most people don't
like this "feature") vs. the WSS Quick Launch bar (it too is limited in
usefulness)
-Topic Assistant

Basically, people would go to their SharePoint portal for the same
reason that they go to an Internet search engine, not to find
information on the Portal, but to locate information that the portal can
point them to:
1. Search for content that they aren't sure where to find or 2. Browse
for content that they aren't sure where to find

I like to say, people don't typically go to Google to see what Google
contains, people go to Google to find other sites to visit that contain
what they are looking for. Google is just the doorway to information,
not the house.