mainly, the rust at the bottom of the air handler, and the copper gas line that goes into the furnace. Eventually, the metal cabinate on the furnace, will rub through the copper and leak. I take it you have propane, due to the regulator + copper.
The lack of a drip leg.
Technically, you should also have a propane leak detector up there also, but I wouldn't push the issue.
"Better tell the sandman to stay away, because we're gonna be workin on this one all night."
"Dude, you need more than 2 wires to a condenser to run a 2 stage heatpump."