some ideas for the next restsharp

restsharp2prototype.cs

var http = new Http("https://api.twilio.com/{version}/Accounts");
http.AddUrlSegment("version", "2010-04-01");
http.AddParameter("name", "value");
http.Authenticate = (request) => // authenticators would just follow this contract
{
	request.AddHeader("name", "value");
};

http.Complete = (response, data) =>
{
	// data is dynamic
	// if response content type matches registered handler, it deserializes
	// otherwise it's just the response body as string
};

// for any exception except WebException
http.Error = (error) =>
{
	// handle error
};

http.Post();

Segments, Params and Headers wouldn't work well as dynamics because the names for these do not match the naming rules for C# properties.

opt.Headers.Accept-Charset = "utf-8";  // not valid C#

I like the first version of the constructor. Its a more familiar pattern of string formatting, less typing, and less effort to implement -- although RestSharp already does something similar to the latter.