Maybe it helps to agree on certain roles first and work out an appropriate list per role?

For example, I would see at least these distinctions as useful in writing about required SQL (and by extension) SQL Server knowledge.

Level: Introduction / getting started

* Understanding the basic relational model

* Basic querying (select, order by, group by, joins and sub queries)

Level: Data manipulation / Limited programming

* Point out some modeling features (how to create proper tables, indexes)

* Views

* Procedures

* Updates / Deletes

* Basic understanding of locking and transactions

Level: Programming / Limited administrating

* Most modelling and referential integrity features

* Triggers

* Schema's

* Security / Linked servers

* Advanced SQL constructs (outer clause, CTEs, functions)

* Bulk imports

Level: Advanced programming, Administrating

* Add remaining full modelling and referential integrity

* CLR Integration

* Data warehouse features

* Other enterprise specific features

* Isolation models

* Clustering

There is quite a bit more no doubt, i do not claim to know it all. By designating levels/roles and what it is good for, people can quickly get an impression on what they need to grasp and might be missing out on. It could be the best that can be achieved as few will ever get to work with everything and know it all.