We spent 8 months on this project and finally have an initial release for the public. Please take a look at http://www.tagsum.com and please digg our story to help us get started http://digg.com/tech_news/Tagsum_com_launches_with_emphasis_on_social_networking I'm willing to exchange diggs with others who are kind enough to digg our story.
Mainly the back end scalability. We are setup to replicate across multiple database servers, have memcache to store query results in physical memory, etc. The architecture will make sure that it remains very fast no matter the load. We also built in multi lingual capabilities as we plan to launch in several countries and languages very soon. Register and keep an eye o nthe site. We have a lot of unique features coming your way.