Teacher observations

I´ve been participating in threads pertaining to teacher observations, walk-throughs, etc., and recalling previous conversations with colleagues regarding the same, and still have reservations with the notion of using checklists when observing teachers.
How much can be observed when the observer is going down a checklist containing items that are or are not being addressed in class?  

How reliable and valid can an observation be when focused items from that checklist are discussed and predetermined in the pre-observation conference?  

How reliable and valid are observations that are either scheduled or conducted at random when teachers know ahead of time the areas of teaching/learning that administrators find important?
Working together with all teachers in establishing a set of agreed-upon teaching principals should be the bases of post-observation teacher conferences.  Instead of creating a checklist, having a common educational philosophy, mission, and a set of collective commitments paves the way for teachers to chart out their own path in initiating a change in practice.