While updating an older application to the latest driver, I needed to implement a method which took a connection string and returned a MongoDatabase instance.
I ended up with this implementation:

var url = MongoUrl.Create( connectionString );
var serverSettings = MongoServerSettings.FromUrl( url );
var s = new MongoServer( serverSettings );
var db = s.GetDatabase( url.DatabaseName );
return db;

However, after deploying the application to production, we discovered that it was slowly leaking memory. After some investigation, I found that after calling this method 500 times, a number of objects had been created, which would not be garbage collected:

MongoServer is not disposable and I cannot find any references in the documentation about how to "clean up".
I am now using this implementation, which does not seem to leak:

var url = MongoUrl.Create( connectionString );
var client = new MongoClient(url);
var s = client.GetServer();
var db = s.GetDatabase( url.DatabaseName );
return db;

However, GetServer() is deprecated, so I expect it will go away at some point.
I realize that I "should" be using IMongoDatabase instead of MongoDatabase so I could just use client.GetDatabase(...) but this is a large code base, and the API of IMongoDatabase is radically different (and worse IMHO).