Many times, I get question from my customers if they should build a new application in SharePoint or ASP.net. For certain kind of applications such as Collaboration or KM Portal, Internet facing web site, intranet social networking site, applications requiring browser enabled forms or Enterprise Search, SharePoint is THE option to go for.
However, it’s not a straight forward answer for all types of applications. You need to look at different aspects before deciding. I’ve put a high level list of questions that I use to make such decisions. Hopefully, the questions should help you also.