the most important aspects for a point and click type adventure are unexpected humour, fantastic puzzle elements, changing scenery (so a single area can look multiple different ways), and a completely original story/ambience. these are all reasons that The Neverhood is the greatest point and click adventure ever made, and, if it weren't so short, would be the greatest game of all time. Machinarium is also good at all of these, though to a slightly lesser extent.