The first thing is that why do this instead of using Google to find out the most popular niche sites? If I type in "restaurant forum" in Google, surely the first 4-5 pages will contain useful information, no?
Secondly, people have to remember that this list exists when they are thinking of ideas. First thought in anyone's mind is to Google "restaurant forum". You aren't going to overcome that line of thinking in this community - you just aren't. The only hope you have is to augment it by providing information that Google doesn't offer. What's the chance of that?
Third - let's assume that you get a lot of interest on this thread and people submit 500+ links. Awesome :) Who will take it upon themselves to categorize these and present them in a helpful way? Surely you wouldn't expect anyone to visit a page with 500+ links on it and search one by one through the links to see if one meets his/her needs, would you? If you did a page search for "restaurant" on this page, a site like http://www.fohboh.com/ would not show up b/c it doesn't have a descriptive URL or tagline next to it. So someone has to take this list and make it usable. Who will do that?
Fourth - who will curate this list to ensure that only the "right" sites are added (rather than someone submitting their own site for SEO purposes)? It would have to be a mod b/c OP cannot edit others' submissions. Can you get a mod to agree?
Lastly, who will curate this list over time by adding, removing, and editing it? New sites are always on the horizon and thus will need to be added over time. Current sites will die or fade in popularity. Someone will have to be on top of it. HN doesn't have "sticky threads". Even if it gets voted to the front page, it will eventually/soon fade to the 2nd, 3rd, etc. Only a certain number of stories are maintained in perpetuity (at least in the "More" button). How will you ensure that it stays alive?
Sorry - the "actual benefits received" vs. "how much work will be required" just makes it untenable to me.