when I define a table via adminer I cannot set a column to be "not null". On the other hand if I set a column to be "null" and reload the table structure the checkbox for "NULL" is unchecked so I never can remove the "NULL" from that column again. That's just some tiny ui bug but limiting adminer a lot.
The problem with not null is when you define a column which is not of type int and a primary key then you can insert empty primary keys. I defined a table "makes" with the primary key "name" which is a varchar. As I cannot set it to "not null" I can input empty data (yes, I'm using innodb). For me this isn't a problem but it's not me entering the data in to the database so it's a crucial requirement.
Hope it helps to improve adminer which I think is a great alternative to phpmyadmin. It's also pleasent to use on the ipad where the current phpmyadmin 3 doesn't work.