The most of the money goes to link building. Also a good amount goes into, a unique design and also to some useful mods, that will add useful features to your directory.
There is a ton of great information in this forum - the most important thing I'm taking away from this is the advice to build links. I am constantly trying to understand the relationship/ratio of inbound vs outbound links - but as mentioned in this forum a couple of times, I won't worry about building PR until later. Cheers!
and willingness to learn from the more experienced in the industry The Tree has to be planted in a good neighborhood too...
Guaranteed way is to add 10 quality links by self on every 100 user submissions. Add leaders of every niche to motivate others.