I've just taught a SharePoint governance class and thought what is the key for successful governance of SharePoint?

So people keep their jobs and the business runs its operations on SharePoint with confidence.

Assign ownership and accountability for IT governance. 

Like any major initiatives, IT governance must have an owner and unaccountability's. Ultimately, the board is responsible for all governance, but the board will expect or delegate an individual (probably the CEO or CIO) or group to be accountable for IT governance design, implementation, and performance—similar to the finance committee or CFO being accountable for financial asset governance. In choosing the right person or group, the board, or the CEO as their designate, should consider three issues.

First, IT governance cannot be designed in isolation from the other key assets of the firm (financial, human, and so on). Thus the person or group owning IT governance must have an enterprise-wide view that goes beyond IT, as well as credibility with all business leaders.

Second, the person or group cannot implement IT governance alone. The board or CEO must make it clear that all managers are expected to contribute to IT governance as they would contribute to governance of financial or any other key asset.

Third, IT assets are more and more important to the performance of most enterprises. A reliable, cost-effective, regulation-compliant, secure, and strategic IT portfolio is more critical today than ever before. The person or group owning IT governance must understand what the technology is and is not capable of. It is not the technical details that are critical but a feel for the two-way symbiotic connection between strategy and IT.

The CIO owns IT governance in the majority of sizable firms today.  Other enterprises have chosen either another individual (the COO or occasionally the CEO) or a committee (say, of senior business and IT leaders) to own IT governance. I have not observed any one approach that always works best. It takes a very business-oriented—and well-positioned—CIO to deliver on the first consideration and a very technically interested COO or CEO to deliver on the third. Committees have the problem of meeting only periodically and dispersing the responsibility and accountability.

Most CIOs will then create a group of senior business and IT managers to help design and implement IT governance. The action of the board or CEO to appoint and announce the CIO as accountable for IT governance performance is an essential first step in raising the stakes for IT governance. Without that action, some CIOs cannot engage their senior management colleagues in IT governance. Alternatively, the board or CEO may identify a group to be accountable for IT governance performance. This group will then often designate the CIO to design and implement IT governance.

Remember, what gets seen gets measured

 This is also a good post:  Link