I can see hundreds of new 'directories' springing up in the next few months.
Rationale: I could set up a directory - have a few good ranking PR pages from my own network point at it.
Set up a post your own link in any category with as much descriptive text as you like - so that webmasters can post as many websites as they like in any category.
This would need no management / administration from me and the directory would grow unchecked - I would not have to worry about linking to bad neighbourhoods and I would not leak any PR.
I would have lots of pages crammed with keywords for the SE's to munch on.
I wouldn't care about the quality of the site - I'd put some contextual advertising on each page so that I could catch the traffic that got directed from the SE's.
I should point out that I would never actually do this - but know from experience there are thousands out there who would (and will).
Thus, I think if you operate a directory and use the nofollow - you will cease to be an authority.
In the long run, I suspect this could really hurt you as G and the other search engines will start penalising directories that do this.