There are lots of features, but if you are not using them, do you care? Maybe, but it might be difficult to test some things (replication) if you are not set.
I'd check connectivity. Use QA, isql, osql, and ADO (OLEDB) to check connectivity. You can also run netstat from the command line and look for a listener on port 1433 (TCP/IP). I'd also go to remote clients and check named pipes and MP if you use them.
Create a db, add objects, query them, alter them, be sure things are as you expect. You could build a simple script for this.
Backup and restore your db. Be sure this works.
Run DTS and make a package, be sure you can save it, execute it, and it works. Again, build a standard that tests things you need and save it.
I'd check mail and sql agent. Schedule jobs and be sure they run. Check local as well as remote connectivity for Sqlagent (if you need it). I don't use Operators or Alerts, but you might want to check these.
Check IIS if you need the XMl links.
I'd also check Windows and SQL authentication. Be sure both work.
Full-Text search if you use it.
the best test for me would be to restore an exsting db to this server and check the applcation to see if it works.