The most important feature I came across is multiple files usage. My current editor (TexStudio) has a neat feature where I use my 'main' document as some sort of a container. I put the document header (with the packages and everything) in there and for every chapter I use a different file. This lets me work on a chapter in a seperate file which does not get me lost in a 1 huge file when looking up stuff in the content.
It also shows a tree of your document structure per chapter/section/subsection/subsubsection so you can navigate easily through the document. This might be a useful feature for this online alternative.
Also, compiling from one of the chapter files actually compiles the entire document, which is nice (because otherwise it'll complain that is has no document structure, packages, etc).