i'd just like to chime in with another duplicate response: the nexus fans are appreciated so much because the quality is constant. sure there are other popular fans, most of them much cheaper, but you have to buy several of them to find one that's suitably quiet. right now i have 20 or 30 80mm fans, none of which are as good as a single nexus would have been.
i'm not sure whether anyone knows how nexus performs their quality control - either they're buying fans as cheap as everyone else and cherry-picking in nexus headquarters, or they're paying the manufacturers more per fan for better qc at the manufacturing plant. whatever they do, that's what we're (happily) paying a premium for.
for all sizes:
- 25mm thick models are essential, but a 38mm-thick 120mm fan would be nice.
- fan material doesn't matter, so long as it isn't noise-inducing (vibrates, resonates, etc.)
- fan color or colors doesn't matter, it could be pink for all i care.
- bearing type shouldn't matter, but ball bearings get louder over time (right jafb2000?) - sleeve or hydrowave bearings would be better.
- rpm should be around 1400 for 80mm/92mm, and 1200 for 120mm. those won't be silent, but are all the headroom i would find acceptable.
- temperature control would only be useful for me if it was on/off like some nidec models - the fan is completely stopped until the thermistor reaches 55 degrees c or thereabouts, then it spins to full-speed. no ups-downs to worry about. thermistors should always be on the end of a lead.
- rpm signal output is useful for me, alarms and auto-restart are not.
- power connector should be dual 3-pin and 4-pin, so motherboard-based control is possible but 5-volt modding is also.
- lead length should be 18" or so, it can always be zip-tied later.
- lead colors or sleeving doesn't matter.
- led options, uv or other purely-cosmetic functions don't matter either. the only such addition i've liked has been the heavy extruded aluminum frame that some powmax fans come with - the frame does a good-enough job of vibration dampening that the fans can be hard-mounted with little noise.
now as for other non-standard features. the most important point of all would be quality control. it won't matter if your fan has the best specs in the world if it ticks, buzzes, clicks or makes grating noises - i don't know which link in the chain has to perform the quality checks but it has to be done, this is essential. otherwise it's just another quiet-ish fan, of which we already have many to choose from. also i guess the fact that it's going to be a low-rpm fan means that blade designs could be tweaked to improve air pressure and flow rate - perhaps once you're sub-800 rpm it becomes advantageous to use 16 or 20 blades, who knows. i certainly doubt that a particular set-in-stone blade design works best for all rpm ranges.
basically we want to move air and do it silently - some people shoot for 20 dba, others 15. start by finding fans capable of such a noise level free of any motor noise or vibration, and from those fans see what you can do to get the most airflow without making them any louder. once you've found the best air-pusher for those noise levels, put them up for sale