A few things I've seen with null values:
* You're storing multiple types of data in the same table (often paired with a column indicating type). Some types have nulls in a column, other types have non-nulls.
* You've created an arbitrary limit on the data. Ex: a books table with author1, author2, and author3 columns.
* You add complexity to your queries making them less maintainable. For instance, you might start injecting COALESCE, IS NOT NULL, empty string checks, etc. all over the place.
* You need to be more careful with math. Ex: what is null + 10?
Null isn't universally bad, but it comes with issues you need to be aware of.