We were going to wait until we'd fleshed out the examples a bit more. But since you need it now we've uploaded a simple example ASP.NET project (for C# for now - we'll be adding VB.NET later). Inside you'll find a file LimeLM.cs with simple functions wrapping up the details of the API interface.
You can just use that instead of rewriting it yourself. That being said, here's the answer to your question:
"feature_name[]" and "feature_value[]" both are strings. You simply add feature_name[] and feature_value[] to the paramater list for as many features as you want to set. For example:
...?method=limelm.pkey.generate&feature_name[]=first_feature&feature_value[]=professional&feature_name[]=update_expires&feature_value[]=2012-06-25%2004%3A13%3A16
This parameter list will generate a key with 2 features: "first_feature" (with a value "professional") and "update_expires" (with a value of "2010-06-25 04:13:16").
Tell me if this is helpful.