Lets just take a quick look at a set of related abstractions:
electronic signals (yeah that's right go fuck yourself)
Ok seriously, I'll try to list what's involved with just data access:
- Set and Relational Theory
- Relational Database Topics (schemas, functions vs stored procs, views, transactions)
- Sql Server
I think that hits the big ones involved for a general .NET data access stack. Now working at the NHibernate level that abstracts away a lot of ADO.NET (provider creation, connections, transactions a little, generating some sql), but you aren't exempt from knowing about it. Beyond that you can't ignore that fact you have a real database underneath that speaks SQL and has its own way of doing things (Sql Server vs Oracle for example).
That's something everyone has to keep in mind: working at one level of abstractions does not mean you can be ignorant of lower levels of abstractions. I do differentiate between the required knowledge to work at a level of abstraction and potential knowledge.
Taking my previous example if you are working at the NHibernate level you are required to know basic NHibernate and some of it's abilities. You are also required to know SQL and something about relational databases (a little design mostly). Now there is the potential (and advantages) to knowing the inner workings and reasoning behind NHibernate (such as Unit of Work, advanced relational mapping), how to work directly with ADO.NET (providers, core interfaces, managing connections), plus all the things you can learn about how your database of choice works.
So what does this mean for people coming into this field... I hope someone is looking out for you. Seriously, you should try to get in at some level of abstraction with a mentor. Realistically you'll have to put some effort into finding a mentor. They can guide you into understanding the level of abstraction you'll be living at and figuring out how it fits in.
What does this mean for people already in the field? You should figure out the potential knowledge in your stack of abstractions you work with (both above and below where you are). Figure out where in that stack you live and try to tackle at least 2 levels down to increase your understanding. I would also suggest going up a level to keep in mind how something is used, but the important thing is to not be ignorant of what's powering your level.