As a mechanical engineer, the lift fan was really impressive to me. The reliability on that thing has to be awesome, because a failure during hover will almost certainly destroy the aircraft and will probably kill the crew.
Also I loved that huge titanium bulkhead on the X-35. What an awesome piece of machining work. Can you imagine the value of the recycled titanium chips for each one of those they make?
I tell you though, looking at the stress those engineers were under, and seeing the look on the losing team's face, makes me not miss my old job at all.