Junckers major flaw is that he believes in the European Union. Cameron doesn't. Cameron wants all the benefits of being a member, but he doesn't want to follow the rules.
Junckers wants to take the EU forward, Cameron wants to take it back. Cameron wants to renegotiate the Maastricht Treaty which we agreed to 22 years ago.
The funny thing is, Cameron has not said what changes he wants. He has made some broad statements but nothing specific. We do not know what the British position is. If we are not being told what we are trying to change, how can we tell if the renegotiation has been successful or not.
Junckers is a trained lawyer and his ability to hammer out a deal is well known. When trying to get Britain to join the €uro, Junckers proposed 19 different deals to Tony Blair, who fortunately rejected them all.