Why is the Unit of Work pattern considered fast and efficient when working with an ORM

Please note that I only have minor experience with the .NET framework and that this answer only relates to the architecture part of your question.

As far as I understood it, you are basically applying the following architectural patterns in your application:

Layers: It seems you have a persistence layer (Repository Pattern), a business logic layer and a view layer.

Model-View-Controller: This pattern is applied in the view layer using ASP.NET MVC.

I guess the question is: what does a BLL look like and what does the MVC controller that uses it look like?

First of all, this architecture is intended for large-scale, enterprise applications as Robert Harvey mentioned in his comment to your question. What is characteristic for such systems is that the domain logic (which you encapsulated in the business logic layer) should be accessible through various interfaces.

Consider Twitter for instance - Twitter could have a business logic layer which provides services for registering and validating users, posting tweets and much more. On top of this business logic layer, multiple other components could exist in the view layer, e.g. a web interface and web service component. Since all business logic is encapsulated in the business logic layer, you can follow the DRY principle and improve maintainability and other quality attributes.

To come back to your question - you should encapsulate business logic and data access in the BLL. The controller (think MVC) should make use of the BLL and should not have direct access to the database. Consider the view layer as an interface to your application.

So I feel like if I have a BLL, I should be passing it the IUnitOfWork, so that it can use that to create the IRepository instances that it needs. But how will the BLL (separate DLL from the front end) 'know' what implementation of IRepository to build?

The BLL should know which repository (data source?) it uses. The layers above shouldn't control this. Again, consider the view layer only as an interface to your business logic.

If you don't need the overhead of a business logic layer, you may also choose to use it solely for data access in form of data access objects.

I hope this helped clarifying the various components' responsibilities.

I just finished an article today about the repository pattern (with sample implementations).

The thing with most .NET ioc containers is that they support scoping. That is, they can create objects with a limited lifetime. That works very well with HTTP applications since the scope is the same as the lifetime of a HTTP request.

If you use ASP.NET MVC you can combine that with built in features of MVC to trigger the UoW if no errors where detected: http://blog.gauffin.org/2012/06/how-to-handle-transactions-in-asp-net-mvc3/

If you use any other kind of application I usually create a scope by myself (for instance to wrap a command):

using (var scope = MyContainer.CreateChildCointainer())
{
    using (var uow = scope.Resolve<IUnitOfWork>())
    {
        // do something here


        uow.SaveChanges();
    }
}

The thing is that the repositories etc do not have to be aware of the unit of work (if you use databases in .NET). The UoW implementation can make sure that all OR/M or UoW implementation enlists all db commands in the transaction.