We had a good discussion on the NameValueCollection internally not too long ago and I asked my friends on the BCL Team to post a summary of the things that were discussed.  Kim Hamilton delivered and how.  Great tidbits like:

For example, when doing 40,000 lookups on a collection size of 100,000, NameValueCollection is about 2.6x worse:

NameValueCollection time: 0.050 sec
Hashtable time: 0.019 sec

 Check out the full article here.