Its a bit hard to say if there was one particular issue that has changed my mind on the subject, bearing in mind that my position on climate change has evolved from on of support for the GW lobby towards the opposite perspective over a period of 5 or 6 years.
I've gone the other way: starting out as a skeptic and gradually came to believe that global warming was happening and it was probably mainly due to humans burning of fossil fuels.
I came to this belief for various reasons:
1) We know C02 is a greenhouse gas. We know that human activity has increased the net amount of Co2 in the atmosphere. We know that the temperature has, on average and on a worldwide scale, been increasing. Correlation does not equal causation, but sometimes it is a pretty good indication, especially if you have no other likely causes.
2) The experts are remarkably consistent in their acceptance of the basic facts, even if they disagree about total effects and implications.
3) The skeptics keep bringing up the same arguments even after they have been disproved. Sunspots and solar cycles, for example (see RealClimate.com).
4) There is more to be gained for new researchers from denying AGW that there is from confirming it. It is argued that some established scientists have staked their reputations on a pro-AGW stance and would be reluctant to abandon it, which is probably true. However, other scientists who are not so well established would get a LOT of renown and remuneration if they upset the applecart with a REAL alternative theory.
I would be prepared to change my mind again if the evidence pointed that way, e.g. if peer-reviewed, replicated experiments done by reputable establishments showed a huge increase in volcanic activity that overwhelmed the effect of human-produced CO2.
Equally, if medical researchers showed conclusively that viruses and bacteria did not cause disease, I would take that seriously, but until then I'll keep washing my hands when necessary.
Question to the skeptics: what would make you change your mind?