Hi Fractal - one of the Overleaf founders here. SL is great, and had a very nice editor, but there are a few differences between it and Overleaf:

- Rich text mode on Overleaf makes it a lot easier to collaborate with colleagues who would otherwise be put off by a wall of LaTeX code.

- Automatic compiling on Overleaf - you don't need to manually hit compile, we do it automatically. This makes it easier for authors new to LaTeX as errors can't build up

- Unlimited collaborators - share the project with as many people as you like.

- Direct links to publishers to help streamline the submission process for academic papers.

Feedback always very welcome! Lots of different ways to approach this, and we're continuing to build and improve on the service.