A dangerous novelty is something that people are attracted to, but could potentially be toxic. For example, the alcoholic drink absinthe seems like a noble drink, but if too much is consumed, it can make drinkers very ill, thanks to the poisonous tree wormwood.
A person who is slick and savvy enough to woo people to join their cause, only to discover their ulterior motives are detrimental, is a dangerous novelty. Dictators have gained power through charisma and smooth talk, only its people are then ensnared in poverty, misery, and the endless threat of violence.
In the eight years as President, Barack Obama turned from a historic novelty to a slick politician. For all his charisma and rhetoric, people actually think he's still the solution to the nation's ills, if you consider capitalism and Christianity immoral. His British counterpart is poised to take over the Labour party and introduce plans that would make old school Communists leap onto their tables and lustily sing Le Internationale.
When we're fed up with the status quo, we turn to the comfort of the charismatic, the ones who say, "Follow me and we'll (fill in unrealistic, but tailor-made promises here)!". The only problem: they find bugaboos and scapegoats for the masses to revile, hoping that enough lies sweep them into power. Once in, those tailor-made promises evaporate, and the path to misery, regret and resentment begins. Freedoms erode, indoctrination goes into hyperdrive, and dissent is snuffed out, first by ostracism, then by force, and finally by torture. The only escape is death.
Should we avoid dangerous novelties? Yes - if the maxim "if it's too good to be true, it is" holds, then those who are skeptical have every right to be and warn frequently; those who are blinded by dyspeptic populism and huge amounts of envy must be forced to witness those who were victims of that same rosy glassed look.