.
IMO, the iMac slim box design lacks efficient inner ventivation to keep inner parts cooler - when iMac is used for ABOVE NORMAL usage loads. And, their energy star rating makes their factory inner fans run too slow - which doesn't help either. Compared to an open cavity box design, the iMac insides run hot. It's the side effect of their thin box design.
During their next box design, wish their box was 1" thicker and it has more inner ventilation fins. And, better ability of natural cooling air flow - in case one of its inner fans become defective (and not noticed).
3rd party fan control (like SMC Fan Control and iStats Menus) allows one to set the iMac inner fans faster. Thus, allowing the inner chassis to operation slightly cooler. Faster the fans means insides are cooler. However, faster RPM of its inner fans means the fan noise become louder. In some environments (like music recording area), faster RPM / louder sounding fans is a bad thing. Besides this, some folks don't effiently "tweak" the inner fan RPMs properly. re: they don't tweak the fan RPMs +/- every other week to see what works best - for their unique situation. They set to super fast RPM and leave the setting. Thus, making more fan noise then needed.
To view the inner cooling details of my iMac, surf:
http://i178.photobucket.com/albums/w251/Spike99-Pictures/New iMac system/FanSpeedJune25-2011.png
http://i178.photobucket.com/albums/w251/Spike99-Pictures/New iMac system/InternalTemps2010-08-30.png
Some folks don't like 3rd party software to tweak factory inner fan speeds. For these folks, they purchase a "near silent" 6" desk fan and place behind their iMac. And, this fan's different speeds allow one to turn up or down its noise level during sound recording moments. For example:
http://img03.static-nextag.com/image/LORELL-44551-Desk-Fan/1/000/008/244/839/824483983.jpg
Personally, I don't like driving any item (like an automobile or computer) until it automatically shuts down or its dash light comes on (from inner heat over load). As some say, the "too late" dash light. Too late because its inner "over heat" damage is already done. To me, applying "proactive" low cost improvements to keep an iMac cooler is a good idea. Especially a low cost external fan or low cost internal iSlayer iStat Menu program. Or, a combination of both cooling improvements. To me, being "proactive" is always better than being reactive (which is often too late).
I also recommend that a iMac chassis should only be covered AFTER sitting in Power OFF state for 24+ hours. Thus, ensuring the iMac insides are cooled down. Covering a HOT item with a thick blanket (sort of speaking) is a bad thing....
Hope this helps in your reasearch....