After using LaTeX for the past ~8 years one thing I have noticed is that instant preview is not adapted at all: LaTeX compilation is simply too slow. I prefer compiling when I want to see how things turn out after making a bunch of changes rather than having the compiler throw out a bunch of errors — and breaking my workflow — just because for some reason it decided it should recompile while I was in the middle of writing a macro.
Another minor quibble I have with these kind of services is that they often only support [pdf]latex, I'd like to be able to use [xe|lua]tex (after having a taste of fontspec/unimath I simply can't go back).
I'd love to see a live-collaborative LyX... I'd like to see a live-collaborative everything, but most open-source codebases make so many assumptions about a single editor per buffer/document, or a single cursor per buffer/document, that adding collaborative functionality is a gargantuan and very application-specific task. I don't think it's even possible to find a general strategy for making the necessary modifications, but if someone were to figure out how, they'd be the most in-demand consultant in tech, that's for sure.
The 'compile in the middle of my macro' problem is a tricky one. writeLaTeX won't interrupt you with errors while you keep typing, but that's the best I've managed so far.
I find the auto-preview useful when I'm making tables or figures. It also enables some cool stuff, like this 'workbook'-like document that I recently did with a friend:
http://writelatex.com/4134bzfwng
There should be probably be a way to turn off the preview, though.
It doesn't necessarily, but I find myself spending too much time tweaking visual appearance and layout if I compile too often. I prefer (if I can manage the self-control) to write up a semantically oriented LaTeX document and then only do a round of layout-tweaking (e.g. babying the figure-placement algorithm) at the end.
I really wish there were Latex editors that would preview just the equation you're currently editing. I know macros make that tricky, but it would be a killer feature for anyone getting started with Latex.
I believe that AUCTeX [1] does essentially this in EMACS. It embeds a preview of each equation into the buffer, but I don't know whether it's smart enough to handle macros.
Another minor quibble I have with these kind of services is that they often only support [pdf]latex, I'd like to be able to use [xe|lua]tex (after having a taste of fontspec/unimath I simply can't go back).
Other than that, great project!