Mark Suster Partner Upfront Ventures and former entrepreneur
Suster identified “ five skills [angel investors need to excel]”:
1. “Access to the Best Deal Flow” 
2. “Domain Knowledge”
“Domain knowledge  [is] [well-honed industry knowledge].” “ It requires domain knowledge to know what you’re talking about and success long term as an angel.  One of the biggest problems is when “you don’t know what you don’t know.””
“ Relationships with VCs  protect [angel] investments” by taking out angels’ positions.
4. “Deep Pockets”
A company is either acquired or has an IPO in an average 7-10 years. Suster says that angels’ deep pockets providing follow-on investments minimize these risks: dilution, inability to protect good investments and a lack of deal diversity. “ [Angels] need to be able to do a large enough number of investments to create enough deal diversity.” The greater the deal diversity, the lower the risk.
“ the best investors influence their end-games through well-cultivated relationships with eventual buyers of their portfolio companies.”  [Helping] companies exit to the right buyer and importantly at the right time” can be crucial. Mark Suster, Angel Topics Sept. 14, 2010 http://www.bothsidesofthetable.com/angel-topics/