Most BD roles are either sales or support/partnership driven, some with quotas to meet. Both sales and BD require selling in one form or the other. So then, what differentiates sales from Business Development? In my opinion - All Business Development people are salesmen; all salesmen are not wired to do Business Development"

The ideal BDM is the one who can visualize opportunities from a sales, product, finance, development and operations perspective. At the macro level, the BD person is a jack of these trades, and a master of putting it all together to identify a new revenue or strategic opportunity.

I would look for the following skills/traits in a great BDM in a startup, for a technology product:

Sales: Is the person able to confidently present, communicate, follow-up and do what's necessary to close a partnership?
Industry Knowledge: Is the person up-to-speed on what's going on in their particular industry? To what level of detail? Does he have an opinion on What, Why and How of the current happenings, and future possibilities?
Technical Aptitude: The person should be able to comfortably present the possible implementation of the discussed partnership at a macro level, and get the product team involved at a stage when there is a verbal agreement on terms, and implementation needs to be finalized.
Curiosity: There needs to be a genuine curiosity about people, things, and happenings. This can be judged by the type of questions that the person asks when assessing a situation or a potential partnership. The ability to gather intelligence, and not just data is really important.
Numbers/Spreadsheet: There are x # of ways you can structure a deal from a financial standpoint - does your BD person have the required mindset and skills to frame up various revenue, operational or strategic scenarios?
Strategic vs Tactical: Maintain a balance of thinking at a macro/company level, and executing at the micro/departmental level. Should be able to follow and/or define the rules of engagement between different departments.
Contracts: Should know basic legal terms, and identify red flags within T&Cs, Privacy Policies, and other legal documents.

The distribution of time between various aspects of the BD role would vary, but at the end of day, I expect my BDM to be the relationship person who can identify, close and/or manage partnerships, on demand