Some random thoughts about directories....
I think that the directories that are going to be valuable to you are that ones that are likely to send you traffic... which is to say, the directories that have lots of engaged traffic themselves and mesh well with your site or business.
Yes, "value add" is generally a large part of that, but what that value might be depends on your niche, and on the type of directory you can expect in that niche, and the kind of business you operate in that niche.
For some niches, a directory that's simply the curated list of sites or businesses suffices. In an age of databases and data mining, that's becoming increasingly rare... but for some niches, that's maybe all that the traffic volume currently justifies.
For other niches, you'll need directories with ratings, user comments, maps, lots of buzz. Some of these are essentially forums with summary pages.
If the directory is a list... with whistles and bells or not... how complete is it? How easily can you find the information you need? If you put yourself in the shoes of a potential customer, would you be happy if you were using the directory to find a supplier like yourself? If not, maybe that directory isn't for you.
Are you a wholesaler, retailer, manufacturer, service provider, consultant? Local, national, international? B2C? B2B? How do you fit into the ecology? (If you are effectively a directory yourself, you may have a hard time getting listed in other directories.)
If the directory has reviews, do you trust the reviews? Are the reviewers essentially anonymous, or do they have reputations? While today the hot directory for local restaurants might be Yelp, is there something coming up that's going to become more trusted over time? Where do, say, Facebook and Google+ fit into this picture?
I see the landscape evolving, with the old model of a directory as a phonebook long gone in local, and certainly changing in all areas, but not quite yet crystallized enough for the shape of the new formats to be clearly visible. For many niches, there may not be any currently viable directories that are obvious, but there may be something equivalent, say in the form of communities, beginning to come together.
