On the subject of density altitude (and with temps over 40 dec C here in the summer it's something we need to be aware of) as well as the changes in take off, landing and climb performance there is the issue of leaning the mixture.
When I was a stude in the UK I was told "ooh no, don't EVER touch the mixture control below 5000 feet!" Visitors to Aus with their JAR licences are, it seems, still being taught the same nonsense.
Warrior POH says "the mixture should be leaned in cruising operation when 75% power or less is being used" (anyone here ever got MORE than 75% power in a Warrior?!)
It's amazing how people will disregard the POH "Because my instructor told me...." or have never seen or read the POH (something to occupy them while the instructor will be "With you in a minute") which is a failing on the part of their flying shool.
I wonder if fouled up plugs could have contributed to the IOW tragedy?