It's silly worrying about things which are out of your control.

这个句子不针对具体某人，只是泛泛而谈，用动名词作主语。

Nick is worried that He can't get promoted, but I think it's silly to worry about things which are out of his control.

这个句子silly是针对Nick担心这件具体的事例，即某人“蠢”体现在哪件事情上，要用不定式作主语。这个句子实际等于：

It's silly of Nick to worry about...

Nick is silly to worry about...

所以本句不能用动名词作主语。