I am however quite clear that there are many other factors in starting a new web-based application. Many of them are mentioned in Startups in 13 Sentences.

I especially agree with the second sentence on the list. Start up quickly! I believe this to be true, in the sense that from when you begin the project untill the beta is released, and actual users begin to use it, there should be as small an amount of time elapsed as possible. The reason, is listed in the list as well, is that when you do as 37 Signals would want you to, you could end up doing all the wrong things.

It is just like the actual development of software. Test early and test often.