What makes a good PLA filament? The quality of the filament is frequently mentioned either as the source of an issue but also as the solution to a problem. 

I know that the way you use,store and protect your filament will greatly impact it's quality, especially overtime, but here I'm interested to understand what makes a brand new, out of the air-tight package, good or bad? What to look for when you buy filament (specs,chemical,weight,...)? What are some brands that are notoriously know to make "high quality" filament?
General characteristics of a "quality" filament:


Manufactured to a "high" dimensional tolerance.  Measure the filament in several places along it's length and check for consistency.  I'm pretty happy with filament that has less than 0.05mm difference between its thickest and thinnest diameter.
Lack of impurities.  I've never had a problem with impurities (that I know of) but I've heard and read about it.  Finer nozzle diameters will be more susceptible to clogs caused by foreign matter in the filament.
Good packaging.  You already mentioned it, but good packaging is a sign of a reputable filament supplier.  It should be sealed in a fairly "air-tight" bag with a desiccant to keep it dry.
Technical information available from manufacturer. Quality filament manufactures tend to provide information about their filament's characteristics and other information such as optimal settings, etc.
Good reputation.  This one isn't necessarily fair to newcomers, but there is lots of information online about the reliability and quality of various filaments.  In this case, google is your friend.  (Keep in mind that there will be plenty of complaints that result from poor settings/skill and have nothing to do with the quality of the filament.)


I hope this helps!  :-)