I've really looked into this over the past few weeks, there is a pattern.
The micro sites that get "abandoned" have a much greater chance of failing than those that at least get some link building from time to time.
Refreshing the content also plays a part - but it seems that a little link buiding here and there works wonders. If the spider finds another decent number of links of say
PR 6 or better - ones it hasn't found before - then it re-envigorates the site.
It's almost as if, if it finds few/none or poor NEW links, then after a few months the site starts to slide. Almost like a "dead" site.
I'm guessing the logic is "well no-one has bothered to link or bookmark this site anywhere decent for months - must be crap"
The reason people link it to adsense in particular is (if you are like me) you create sites in a "burst" 30 new sites in 2 weeks or 100 sites in a 3 month period.
The links you create to get them going are aging at roughly the same time.
The answer? Well it's to keep a log of your sites, and every 2/3 months pop back and give them some link love.
Scritty
I'm sure the last poster was right about content as well, but new inbound links keep the site alive and fresh