Based on my experience the most common item would be the oxygen is below the serviceable limit and it was not caught until the pilot's predeparture checks. There are many different things that could cause such a delay as speculated earlier, but I know oxygen is a common one.
However it could be one of hundreds of different things. I was once on a 757 that had a weather related diversion to SJC
due to SFO
fog and we were stuck on the ground for over an hour because we "had a mechanical problem that needed to be addressed". The pilots refused to tell the passengers what the problem was. It turned out to be a loose latch on the service door below the first class lavatory sink. It took over an hour because the contract Southwest Airlines mechanic (United had no maintenance staff in SJC
) was slow to bring over his roll of duct tape due. But rules are rules and you can't depart with an open logbook item even if it is for a lavatory sink and a 10 minute flight. In reality they never would have written up the latch if they thought we might be diverting to SJC
[Edited 2013-07-08 16:27:56]
If you have never designed an airplane part before, let the real designers do the work!