I have found these things very interesting and wonder if I have 'enough' understanding of a subject to consider myself as having the basic skills. As mentioned, I do not use certain areas of T-SQL on a daily basis and have to look them up. But I do know when to use them and that they are available, I know what they are used for etc. My definition of core skills is largely depending on the 'grasp' of the subject.

Just my thoughts on this...

(Sorry, this does not add to your list, but might create some questions and answers)