I know the comparison is far fetched, but before del.icio.us there were already tons of ways to keep and manage your bookmarks online - the single user problem had been solved. Yet, del.icio.us added community, the ability to see what others are doing and maybe benefit from their bookmarks if you realize you share their interests, as well as deriving value from the aggregate statistics (e.g. del.icio.us/popular)
While I am not sure whether it is possible to build such an intensive community here in Germany (Yelp is doing fine at it in the US), there definitely is value in community here even if it is a less social experience (see insiderpages.com). We’ll see how much space is left next to the big players…in short, I don’t think the existing services really add enough value beyond pure lookup, and while I just discussed the community aspect above I believe there is a variety of things to make the service more valuable, and through that increase customer retention/lifetime value (more repeat visits…) as well as reduce customer acquisition cost (virality…).

It is all a question of how fast you want to reach awareness (goyellow wanted to be very fast), how different the quality of your service and how viral your product is. (Yelp has been proven to be relatively viral, others in the US less so).
Greetings from Hamburg
Stephan