1. A combined user and topic model with switches for symmetric/asymmetric following and in-site messaging
2. An abstract class called "social object" where you can plug in the code for whatever yours is (video for youtube, Q&A model for Quora, etc.), and things you know you'll need for any social object (such as least-common denominator permissions, version control)
3. Built in FB/Twitter login
4. Built in main stream page with tunable filter controls that can be exposed to users for personalization/customization
5. Fully-fleshed out alerts architecture that can be piped to email
6. Tunable reputation system and spending credits system
7. An API that can do most basic manipulations on things

IMO, there is no good reason to reinvent the wheel each time somebody has a good idea for a social product. Developer/entrepreneurial creativity should be devoted to the ONE part where unique value and differentiation lie: the design of the social object and social transaction architecture.

"Maybe" features:

1. Basic market/auction mechanisms with payment processing
2. Skinning/internationalization architectures built-in
3. Somehow "mobile ready" out of the box

The fact that I can make up such a long list without specifying WHAT the idea is suggests that there is room here for another round of raising the least common denominator.

Things I think will ALWAYS come down to individual designs:

1. Overall look and feel
2. Gamification models if any

(this is a strawman answer to get the ball rolling... I asked the question).