I have a problem with the term "Design" Lead. The word design is just too general.
For me design is a process. It is the choice of a single solution of many many possible alternatives. The job of exploring that design space cannot really be the work of a single person. It takes a multitude of perspectives to identify which is truly the best direction of a design to move.
I need to clarify I am also not advocating design by committee. This is a terrible way to do design and I have seen it in action. Design takes deep deep thought ... and time. However I am not sure you have to be a "lead" to do this thinking. As long as a team is constructed in a democratic manner then respect for this thinking work should be taken as read.
However managing the design process does of course take leadership. For this you need someone with a broad range of perspectives... similar to an architect in building design. That is why I have always preferred the term design/information architect. Part of a architect's role is to synthesize all the information from a multitude of sources into a coherent solution that works.The background of this person and the ability to take on a plurality of views is the key trait that this person needs to have ... and probably a deep understanding of design... they need to be a designer+.
We had a discussion on UX stack exchange chat a few months back about the nightmare scenario where you get a prima donna designer who doesn't understand the art of compromise. Although I am lucky never to have experienced this myself (perhaps because often I was the one taking the designer role within a development team and my views on this are clear) ... I have heard many stories to this effect.
I was once told that a UX person either has to be a Guru or extremely humble. The same goes for designers. Yes we have our guru's: Steve Jobs, Jonathan Ive, Leonardo da Vinci etc... but they are the exception not the rule. For the rest of us I think we need to remain humble and aspire to Guru status one day.
As Aadaam commented below:
If you are humble towards your users and not your fellow developers, you don't cheat them, you help them achieve their goals regardless the effort it takes from you or your team.
This is a great point. Good design requires user advocacy... the central focus of a design has to be "use" ... not features, not cool technology, not super fast algorithms, not marketing value, ROI, not even usability (which I always consider to be a poor term that implies after the fact sticking plaster design). However in order to maintain that focus ideally you need input from user researchers, users themselves, ethnographers... thus our ideal multi-disciplinary team grows even wider. The skill set of our mythical perfect designer even more broad.
So how do we find this polymath designer who has all these skills? The truth is we don't... perhaps that is the value of an approach like SCRUM where the focus is on iteration and multi-discplinary teams. Where everyone has a go at everything. Maybe this way we will develop these people.
I'll get back to you on the case studies... there must some out there.