这是一个同位语从句。that只起到标志性作用,没有任何实际意义。
这句话本来顺序应该是That a great many people are ofen willing to... is human nature. 但是因为主语That a great many people are ofen willing to...太长,而英语习惯头轻脚重,所以要倒装一下,把that a great many people are ofen willing to...放到句尾去。
用such...that句型,并且在Such is human nature之后刻意用逗号停顿,是为了加以强调。这句话可以这样翻译:这就是人的本性:许许多多的人通常都愿意......