I presume that you understand how to create  a lookup column? I'm afraid
that is your only option today.

Relational lists are weak in WSS today (one of the three remaining
platform busters), both for data entry and display. InfoPath can help
with data entry and the Data View Web Part can help  with display. Both
problems can be overcome by a programmer, but out of the box there will
be challenges.

Out of the box, MOSS provides connecting web parts.
You can have a master  web part connecting to a detail
web part.

That is indeed a good option as well.

However, gathering referential information (data entry) and displaying
master/detail data (within a single list  view) are cumbersome at best.

With reporting data, other options may include web
parts using Excel services or SQL Server Reporting
Services. It's still a cumbersome solution.

I'm curious as
to what you think the other two platform busters are.

1. Relational list  support
2. Large list support (on the order of 1,000,000 rows and larger)
3. Native OLEDB/ODBC support for external tool reporting of SharePoint
listitem data without using MS Access as a translator (I'd also like to
see an ASP.NET data provider for SharePoint lists just like they have
for database tables)

In priority order; I also reserve the right to add more busters as I
discover them. My wish list is much larger than my buster list...

What do others think is missing from the WSS v3 platform? That is,
development of applications is hindered because the functionality is not
supported by the platform.

If you want to list a wish (not a buster), identify it as such. Right
now is the best time for me to let Microsoft know what I think their
priorities should be for the next release. Six months from now, WSS v4
will be locked down and not much that isn't already planned will make it
into the next release. So, comment now or hold your peace.