Who Really Cares?
Many decent motherboards have fan throttling circuits to reduce noise, and many of us may overclock but don't care about squeezing the last 3% performance out of a system so ultimately there are three main factors against which the rest are trivia:
1) Wussit cost?
2) Decent long-life fan or cheap sleeve bearing crap?
3) How long's it take to clean the dust out? Some of these sinks are designed by idiots who never bothered to run them long enough to realize you have to pull the entire heatsink out and tediously pick at the dust to clean it, AND that they build up dust faster as well. You can opt for a filtered case to cut down on this maintenance issue, but then you have to clean the filters out all the more often or else replace room-air filters.
I have to agree about the northbridge heatsink issue one person mentioned, in that the ideal heatsink will blow some air towards the heatsink, but I'd have to disagree with another poster that it's necessarily a tradeoff of a quiet fan and needing a 2nd fan, or a louder fan which isn't necessarily true.
Also, most of them cannot have a large case fan effectively pointed at them but they dont need one, if your board and CPU 'sink don't allow for quiet cooling all you have to do is use your preferred method of fan speed reduction for a 2nd fan you strap on.
That fan does not need to be large, there are no motherboards that start out with passive chipset sinks which won't be cooled effectively enough with a sub-1500 RPM, 40 x 20mm fan which is a nearly inaudible if not totally inaudible (once the case panel is put back on) noise level. They simply don't produce enough heat to need anything more unless you're running in a quite inhospitably high ambient temperature, in which case you have to address entire system cooling as merely pointing another fan at the chipset won't cut it.