WriteLaTeX looks pretty cool. Have you looked at using the methods from Kile for better syntax highlighting? I noticed a problem on the (otherwise excellent) main page with the text boxes: http://imgur.com/SojY1, Chrome 23.0.1271.95, Linux, zoom 125% (though that doesn't seem to change it). Also, is Asymptote support planned? (The rotatable 3D figures probably aren't very feasible, but everything else should be.)
We are working on better syntax highlighting. I'll have a look at Kile's, now that you've mentioned it.
Thanks for reporting the issue with the text boxes.
I hadn't seen Asymptote before, but it looks very good, so I'll have to think about it. Thanks for the link! :) (Edit: note that we do support pgfplots, which also makes nice plots.)