Spot Elevations and Model Graphics Style
Just a quick tip here that I always forget about and somehow ends up frustrating me for 15 minutes until I realize what's going on.
Here's what happens: I am attempting to place a spot elevation in a plan view and find that I can only select an edge or endpoint; if I try to select an open area on a floor I get the universal "NO" symbol (i.e. the circle with a slash through it). My next 10-15 minutes are spent checking the associated level, the view range, various properties, the floor object, etc. Then I notice that the view is set to Wireframe.
The moral of the story - if you want to use spot elevations, your view's Model Graphics Style must be set to Hidden Line. I suppose there's some logic to this, but it's not entirely clear.
UPDATE: Okay, a quick web search informs me that I'm not the first person to run across this issue and blog about it.