I worked on the SEO for a directory startup a few years back. They had decent funds to get going and put serious resources into getting the business off the ground, but the result was a multi million pound business that was getting 1 million+ visitors per month at it's height.
During the 4 years I spent on the project I was involved at a high level of the business and saw a lot of copycat competitors pop up and subsequently fail (including one funded by an investor that used to appear on Dragon's Den).
It's a tough market. My former client relied on SEO and PPC to drive traffic, which isn't a strategy I think would scale well these days (post-Panda). But similar budgets could drive traffic from other sources, so I do think it's doable.
The directory model has failed so many times it's a little bit tragic though. DMOZ - spammed to hell (they never reacted to market changing around them and the model didn't scale). Yahoo - died a slow death (again, they never reacted with the market changes). Any link directory - was always spam. Yellow Pages, etc - slow to embrace technology, slow to adapt their model. But I do think a directory model of some sorts could work.
The hard part is developing a business model around the fact that you are constantly serving vastly different customers. But if you can present the information in such a way that it works for the user, regardless of whether or not that's in directory, search engine or another format, then you're on to a winner.
Monetisation is tough. It's hard enough for agencies to convince clients to assign PPC budgets outside of Adwords, let alone spend on a new player. Maybe a model which rewards people for moderating content or submissions might work, but remember the industry we're in - that system would get abused to death.
And another concern is the regionalisation - there are loads of queries where local is good and anything else is superfluous. But all the queries where non-UK results are useful too? I need a hotel, then UK results = win. I want to find SEO blogs = maybe I want the best information, not necessarily the closest.
Or you could simply focus on one aspect of what a SE delivers - i.e. matching the informational needs of a particular group. Nail that niche and expand through different verticals.
All that aside, my gut is that people are just plain lazy. Retaining visitors for something like this is tough - you need a lot of added value features to get people on board and advocating your service. That's one of the areas that my former client did well for businesses (website owners), but not so well for end users. And ultimately you are serving those two distinct groups and it's almost like running two completely different businesses.
There's definitely room in the market for another player though, and it's long overdue IMO. Innovating is the way forward and not just replicating like Bing have been doing. ;)