Not wishing to be overly simplistic, but essentially, China isn't a danger to Taiwan because the Taiwanese are armed to the teeth. In a little more detail, when the nationalists first withdrew to Taiwan, the the communists were struggling trying to hold the rest of China together and recovering from both civil war and Japanese aggression. By the time the PRC was stable enough to look at outward expansion, the Taiwanese were already heavily entrenched. Taiwan has two major strategic advantages. 1. the PRC want it "intact", as it were, thus severely limiting any form of nuclear option, which leads to 2. Taiwan is an island, and seabourne invasions are a really difficult exercise. Until quite recently, mainland China simply hasn't had the industrial base necessary to build the requisite force projection needed. Opposed landing operations require all sorts of equipment the Chinese simply haven't got. Its comparatively easy to arm and basically equip a many million man army. Its quite another proposition to deploy that force into hostile territory and logistically support them for an extended period. Its difficult exactly to say what world opinion would be if an actual invasion took place, China tends to be the elephant in the room that gets away with all sorts of stuff that would meet with instant condemnation and even military intervention if any other country tried it, but I think its safe to say that the probable negative effects on the global economy and Chinese reputation probably plays a part in dissuading Chinese adventurism, too
