How meaningful is the total number of flight hours?
As far as what hours are logged, there are a couple of different ways to do the counting.  On a light aircraft, time on the Hobbes meter is generally used, which equates to "engine running" time.  In the military, it's takeoff to landing, usually plus a small amount for taxi time (which may or may not account for all the time that the pilot is actually operating the aircraft).  In the airlines, it's "block to block" time, so pushing back from the gate until stopping at the gate.

As far as the time all being equal, it isn't.  Not even close.  2000 hours flown from Big Airport A to Big Airport B, 8 hours at a shot, means about 125 takeoffs, approaches, and landings flown, and an equal number monitored.  Almost all of those are radar vectors to an ILS approach -- very standard &amp; predictable.  Five hundred hours as a pilot training instructor probably translates into about 500 landings performed, and many, many more supervised, with a great variety of approaches flown (visual, precision, non-precision, practice emergency patterns, etc)  The "hands on" time on the controls of a military fighter pilot in 500 hours logged is far greater than the hands on time during 2000 hour trans-Atlantic airline flying.

The problem is, "hours" is the currency that we have, and so that's how experience is measured.  Not because it's a great measure, but because anything more granular becomes very complicated and subjective.  Does my 1000 hours flying Lear jets count for more than your 1200 hours flying a King Air?  One's a jet, the other a turboprop.  Did one of us fly 3-hour legs mostly on autopilot on clear days, while the other flew 30 minutes at a time, much by hand, with lots of instrument approaches in bad weather?  Who flew with veteran copilots, and who flew with copilots brand new in the airplane?  Who flew to lots of unfamiliar destinations, and who flew the same canned route(s) day after day?  And how does any of that compare with 3000 hours in a C-5?  Or 800 hours in a crop duster?

All of this to say, that "hours is hours", while it overlooks a LOT of very relevant distinctions between some very diverse types of flying, is still the quick and easy comparison between pilots.  Not because it's wonderfully accurate, but because it's possible (we all log hours, after all), and the more detailed comparisons quickly become hard to objectively make.