R – What would you choose for your data layer today, Linq or Subsonic

You want the Except operator.

var answer = list1.Except(list2);

Better explanation here: https://docs.microsoft.com/archive/blogs/charlie/linq-farm-more-on-set-operators

NOTE: This technique works best for primitive types only, since you have to implement an IEqualityComparer to use the Except method with complex types.

Initial responses (mostly on the LINQ stuff):

• LINQ to SQL is something you can use in real projects; I have done. We tended to have all our Data Access code in the partial class that is generated when you right click on the design surface and select "View Code", rather than scattered as one-liners throughout your code.
• With LINQ to SQL if you modify the database you need to remove and re-add the tables - this is a bit of limitation, with the Entity Framework you can "Update model from database" to auto add/remove these columns.
• It should probably have been called LINQ to MSSQL Server - it is tied directly to SQL Server. If you want to use similar tooling with other data sources, you could have a look at the Entity Framework - however this currently has other limitations - LINQ to SQL was as much as anything a proof of concept that this could work.
• ADO.NET Data Services provide you with a REST based interface to your ADO.NET objects - so you can call simple webservices to retrieve data, without having to write those services.
• No, there isn't a design surface for LINQ to XML - I guess someone could do something with an XSD though, that would be interesting ;)
• You could think of Azure as "An operating system in the cloud", it has a database for storage, although as you state, it's not relational, there's no joins, but you're still going to be querying it for results.

To answer your final question, I've not learnt enough about NHibernate or others to say, but I'd be happy to use LINQ to SQL for basic database access, but I've started looking at LINQ to Entities for my more complex stuff rather than the others - mostly because I like the pretty pictures.