OK, quick one.

I am placing Area Boundary Lines on an Area Plan, using Floors as the bases. And it kept driving me crazy that Revit won't snap to the Floor edges. Why?!

Well, it turns out that Revit automatically turns the option "Apply Area Rules" on, which means that it will only snap to walls according to the set area rules. Turning the option off allowed me to snap to everything again:

In my previous post I talked about the use of snap shortcuts.

Even if you don't use them, revit will tell you what it'll snap to. Not only by highlighting the objects/lines in your drafting view, but also in the status bar on the bottom right:

(Just saw that today and thought I'd share... :) )