According to Nature's submission guidelines:
To submit a TeX/LaTeX file, please use any of the standard class files such as article.cls, revtex.cls or amsart.cls. All textual material should be provided as a single file in default Computer Modern fonts. Please avoid non-standard fonts and packages and remove all personal macros before submitting.
While it is understandable that any font outside of Computer Modern family is considered as a "non-standard", it is not clear what the non-standard packages are. Is there a definition for the "non-standard package" or a list of packages considered standard?
If Nature hasn't made a clear definition or provided such list, would it be impossible to tell whether package X is OK to use prior to submission? Or is it their way of asking to use as few packages as possible to simplify conversion to Microsoft Word?