To much to write here, but I'll try to name few:
- NAME - easy to remember, good domain, suggestive accordingly with your intentions.
- UI/UX - to be attractive and same time simple but functional, adaptive and responsive to user's needs.
- CLARITY - in this way the users to understood in few clicks how it works and what could take advantages from your network.
- CUSTOMIZATION - to give the users a relative control over the interface and modules (to change the theme, to drag the modules in different places, to add/remove modules, etc).
- CLOUD PROVIDER - a good provider which to offer scalability and speed
- SPACE (optional but already become a must) - to provide to your customers cloud storage which to integrates with the main functionality of your network.
- COMPARTMENTALIZATION - as to provide users the possibility to create groups(circles/tribes/etc), to assign different functions to groups/files, to create behavioral rules for files/actions/etc
Depends on you how complex want/need this network to be.
If you consider I could help you draft the architecture of this network, you may reach me at bogdins at gmail dot com or by skype at technobogdins.

You want to add content which goes viral and is engaging for your community. Feel free to call to have your specific questions addressed.

Excellent Question! One that I am currently working towards in my efforts to build a social community around crowdfunding.

A great book I read is by Seth Godin called Tribes. I found this slide share presentation that gives a good summary on the book and the elements described therein should help you on that: http://www.slideshare.net/hhoefele/notes-on-seth-godins-book-tribes-presentation

From my own experience:

1. Needs a structure for the community to communicate to collaborate

2. Needs a common purpose where the members can both further a cause or help each other if that is the cause

3.. Must reward members for positive behaviors and deter members for negative behaviors.

For my own site, I am using http://www.socialengine.com which is a white label social network that is working really well for us. To review what we have built you can check out incubator.realitycrowdtv.com and see the engagement and community we are building.