In MongoDB.Driver.2.1.0\lib\net45\MongoDB.Driver.dll I have found the class MongoDB.Driver.Linq.MongoQueryable whih defines extension methods for IQueryable. I'm missing:

public static Task<List<TSource>> ToListAsync<TSource>(this IQueryable<TSource> source, CancellationToken cancellationToken = null)

Although:
public static Task<TSource> FirstAsync<TSource>(this IQueryable<TSource> source, CancellationToken cancellationToken = null)
is defined.

Workaround: Own extension class:
public static class MongoQueryableExtension
{
public static Task<List<T>> ToListAsync<T>(this IQueryable<T> queryable, CancellationToken cancellationToken = default(CancellationToken))

{ var mongoQueryable = queryable as IMongoQueryable<T>; if (mongoQueryable == null) throw new ArgumentException("Has to be an instance of IMongoQueryable", "queryable"); return mongoQueryable.ToListAsync(cancellationToken); }

}