I don't use many directories: most of them seem to be made from recycled Hoover parts, for sure.
But yellowpages and superpages and Google Local are obvious exceptions: they are absolutely authoritative (compiled from THE source), extremely reliable (their author is used to handling large volumes of data and getting it mostly right, for critical business reasons) absolutely comprehensive in their niche (every business has a phone!).
In addition, they've each put considerable work, in their own way, to make that data more usable: searches, maps, categorizations with a long history of development for their user base.
They have no competition. Oh, sure, there are other startup-type sites that lease and republish data, but (sort of like most ODP clones), their added value is negative. It is the rare clone (like Google) who (by dint of extremely-large-scale data archives and heavy-duty processing) can add enough significant value to compete. The other clones have nothing, nothing but the potential to improve society greatly (by, of course, dying as quickly as is compatible with excruciating pain.)
These define the Ideal for "directoryness." The natural source for a directory is someone who has OTHER motives for knowing that data and keeping it accurate. For example, industry associations, denominational headquarters, government agencies, etc.
Even the largest synthetic "outsider-built" directories (like Yahoo and the ODP) take a great deal of work to maintain, for an inevitable loss in accuracy, comprehensiveness, and authoritativeness. And so, sensibly enough, neither of those try to compete with the PHONE directories.)
And from there, it only goes downhill.
The "commercial" directories (that is, classified ad farms with delusions of adequacy) are on the wrong side of all the economic and data-flow constraints. It SAVES them money not to list some business which doesn't pay -- therefore they are eternally going to be non-comprehensive. With no well-known, reputable publisher and no documented data-collection process, they can not possibly have any authoritativeness. Their interests are opposed to both the listed entities and the public, so they wouldn't be relied on by any reasonable person.
In order to avoid attempts to build perforated low-pressure chambers, what MUST you have?
(1) Authoritative source ("I'm building a new directory, I found your name in another directory and you can pay to be in mine too" is flat out never going to seal a vacuum chamber!)
(2) Local knowledge. If some dude in Eastern Europe or India can clone your data and produce an equally unique site, you all are eventually going to lose out to programmers from the deepest pit of the third world--wherever HTML skills are cheapest. Figure on HTML skills being a free give-away (they are already, for all practical purposes), with the ONLY thing you have that's worth spitting on, is your own unique SUBJECT-RELATED skill and experience.
(3) Added INFORMATIONAL value ("What would you like for your company description? Sure, I'll say it's got the best views in South Berwick with exemplary service and out-of-this-world cuisine!" is just going to cut another hole in the plenum. If people don't see an author as being free to go beyond faint praise and say something bad--or exclude for cause--they'll know it's just an integrity-free shill-for-hire.)
(4) Lots of sweat equity. It's a lot of work to transcribe personal knowledge into any kind of schema. Count on lots of work from the people who have that local knowledge. Or count on being used for a colander, not a vacuum chamber.