This doesn't really classify as a theme, but I love how the action in many tMG songs centers around doorways. There are tons of songs where people stand in/appear in doorways: Nine Black Poppies, Seed Song, Going to Georgia, Raid on Entebbe, Some Swedish Trees, Omega Blaster, Orange Ball of Hate, Alphonse Mambo... I know there are more.
My favorite songs with this imagery are West Country Dream (sure as a surgeon, you slipped your hand into the doorjam) and The House That Dripped Blood (the cellar door is an open throat - I love the sound of those words). It's a simple motif, but it's my favorite.
and, of course, "Going to..."