It would be nice to assign a sticky parameter to tasks and have them appear at the top of lists, before other sort criteria is applied. Yes, you can already do this with advanced sorting and applying a tag or some other task field, but that requires "giving up" that field in one way or another in one's system.

This suggestion should (not) be confused with "sticky notes", proposed by alan74063, which also seems like a dandy idea. (https://www.rememberthemilk.com/forums/ideas/21534/)